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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Population susceptibility to a variant swine-origin influenza virus A(H3N2)
in Vietnam, 2011–2012</title><author><name sortKey="Hoa, L N M" sort="Hoa, L N M" uniqKey="Hoa L" first="L. N. M." last="Hoa">L. N. M. Hoa</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Bryant, J E" sort="Bryant, J E" uniqKey="Bryant J" first="J. E." last="Bryant">J. E. Bryant</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></nlm:aff></affiliation></author><author><name sortKey="Choisy, M" sort="Choisy, M" uniqKey="Choisy M" first="M." last="Choisy">M. Choisy</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff3"><institution>MIVEGEC</institution> (<addr-line>UMR Universités Montpellier 1 & 2</addr-line>,<addr-line>CNRS 5290</addr-line>,<addr-line>IRD 224</addr-line>),<addr-line>Montpellier</addr-line>,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Nguyet, L A" sort="Nguyet, L A" uniqKey="Nguyet L" first="L. A." last="Nguyet">L. A. Nguyet</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Bao, N T" sort="Bao, N T" uniqKey="Bao N" first="N. T." last="Bao">N. T. Bao</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Trang, N H" sort="Trang, N H" uniqKey="Trang N" first="N. H." last="Trang">N. H. Trang</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Chuc, N T K" sort="Chuc, N T K" uniqKey="Chuc N" first="N. T. K." last="Chuc">N. T. K. Chuc</name><affiliation><nlm:aff id="aff4"><institution>Hanoi Medical University</institution>,<addr-line>Hanoi</addr-line>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Toan, T K" sort="Toan, T K" uniqKey="Toan T" first="T. K." last="Toan">T. K. Toan</name><affiliation><nlm:aff id="aff4"><institution>Hanoi Medical University</institution>,<addr-line>Hanoi</addr-line>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Saito, T" sort="Saito, T" uniqKey="Saito T" first="T." last="Saito">T. Saito</name><affiliation><nlm:aff id="aff5"><addr-line>Influenza and Prion Diseases Research Center</addr-line>,<institution>National Institute of Animal Health</institution>,<addr-line>National Agriculture and Food Research Organization (NARO)</addr-line>,<addr-line>Ibaraki</addr-line>,<country>Japan</country></nlm:aff></affiliation></author><author><name sortKey="Takemae, N" sort="Takemae, N" uniqKey="Takemae N" first="N." last="Takemae">N. Takemae</name><affiliation><nlm:aff id="aff5"><addr-line>Influenza and Prion Diseases Research Center</addr-line>,<institution>National Institute of Animal Health</institution>,<addr-line>National Agriculture and Food Research Organization (NARO)</addr-line>,<addr-line>Ibaraki</addr-line>,<country>Japan</country></nlm:aff></affiliation></author><author><name sortKey="Horby, P" sort="Horby, P" uniqKey="Horby P" first="P." last="Horby">P. Horby</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></nlm:aff></affiliation></author><author><name sortKey="Wertheim, H" sort="Wertheim, H" uniqKey="Wertheim H" first="H." last="Wertheim">H. Wertheim</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></nlm:aff></affiliation></author><author><name sortKey="Fox, A" sort="Fox, A" uniqKey="Fox A" first="A." last="Fox">A. Fox</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff6"><institution>The University of Melbourne</institution>,<addr-line>Peter Doherty Institute for Infection and Immunity</addr-line>,<addr-line>Department of Microbiology and Immunology</addr-line>,<addr-line>Parkville</addr-line>,<addr-line>Victoria</addr-line>,<country>Australia</country></nlm:aff></affiliation></author></titleStmt><publicationStmt><idno type="wicri:source">PMC</idno><idno type="pmid">25761949</idno><idno type="pmc">4595856</idno><idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4595856</idno><idno type="RBID">PMC:4595856</idno><idno type="doi">10.1017/S0950268815000187</idno><date when="2015">2015</date><idno type="wicri:Area/Pmc/Corpus">000157</idno><idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000157</idno></publicationStmt><sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Population susceptibility to a variant swine-origin influenza virus A(H3N2)
in Vietnam, 2011–2012</title><author><name sortKey="Hoa, L N M" sort="Hoa, L N M" uniqKey="Hoa L" first="L. N. M." last="Hoa">L. N. M. Hoa</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Bryant, J E" sort="Bryant, J E" uniqKey="Bryant J" first="J. E." last="Bryant">J. E. Bryant</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></nlm:aff></affiliation></author><author><name sortKey="Choisy, M" sort="Choisy, M" uniqKey="Choisy M" first="M." last="Choisy">M. Choisy</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff3"><institution>MIVEGEC</institution> (<addr-line>UMR Universités Montpellier 1 & 2</addr-line>,<addr-line>CNRS 5290</addr-line>,<addr-line>IRD 224</addr-line>),<addr-line>Montpellier</addr-line>,<country>France</country></nlm:aff></affiliation></author><author><name sortKey="Nguyet, L A" sort="Nguyet, L A" uniqKey="Nguyet L" first="L. A." last="Nguyet">L. A. Nguyet</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Bao, N T" sort="Bao, N T" uniqKey="Bao N" first="N. T." last="Bao">N. T. Bao</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Trang, N H" sort="Trang, N H" uniqKey="Trang N" first="N. H." last="Trang">N. H. Trang</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Chuc, N T K" sort="Chuc, N T K" uniqKey="Chuc N" first="N. T. K." last="Chuc">N. T. K. Chuc</name><affiliation><nlm:aff id="aff4"><institution>Hanoi Medical University</institution>,<addr-line>Hanoi</addr-line>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Toan, T K" sort="Toan, T K" uniqKey="Toan T" first="T. K." last="Toan">T. K. Toan</name><affiliation><nlm:aff id="aff4"><institution>Hanoi Medical University</institution>,<addr-line>Hanoi</addr-line>,<country>Vietnam</country></nlm:aff></affiliation></author><author><name sortKey="Saito, T" sort="Saito, T" uniqKey="Saito T" first="T." last="Saito">T. Saito</name><affiliation><nlm:aff id="aff5"><addr-line>Influenza and Prion Diseases Research Center</addr-line>,<institution>National Institute of Animal Health</institution>,<addr-line>National Agriculture and Food Research Organization (NARO)</addr-line>,<addr-line>Ibaraki</addr-line>,<country>Japan</country></nlm:aff></affiliation></author><author><name sortKey="Takemae, N" sort="Takemae, N" uniqKey="Takemae N" first="N." last="Takemae">N. Takemae</name><affiliation><nlm:aff id="aff5"><addr-line>Influenza and Prion Diseases Research Center</addr-line>,<institution>National Institute of Animal Health</institution>,<addr-line>National Agriculture and Food Research Organization (NARO)</addr-line>,<addr-line>Ibaraki</addr-line>,<country>Japan</country></nlm:aff></affiliation></author><author><name sortKey="Horby, P" sort="Horby, P" uniqKey="Horby P" first="P." last="Horby">P. Horby</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></nlm:aff></affiliation></author><author><name sortKey="Wertheim, H" sort="Wertheim, H" uniqKey="Wertheim H" first="H." last="Wertheim">H. Wertheim</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff2"><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></nlm:aff></affiliation></author><author><name sortKey="Fox, A" sort="Fox, A" uniqKey="Fox A" first="A." last="Fox">A. Fox</name><affiliation><nlm:aff id="aff1"><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></nlm:aff></affiliation><affiliation><nlm:aff id="aff6"><institution>The University of Melbourne</institution>,<addr-line>Peter Doherty Institute for Infection and Immunity</addr-line>,<addr-line>Department of Microbiology and Immunology</addr-line>,<addr-line>Parkville</addr-line>,<addr-line>Victoria</addr-line>,<country>Australia</country></nlm:aff></affiliation></author></analytic><series><title level="j">Epidemiology and Infection</title><idno type="ISSN">0950-2688</idno><idno type="eISSN">1469-4409</idno><imprint><date when="2015">2015</date></imprint></series></biblStruct></sourceDesc></fileDesc><profileDesc><textClass></textClass></profileDesc></teiHeader><front><div type="abstract" xml:lang="en"><title>SUMMARY</title><p>A reassortant swine-origin A(H3N2) virus (A/swine/BinhDuong/03-9/2010) was detected
through swine surveillance programmes in southern Vietnam in 2010. This virus contains
haemagglutinin and neuraminidase genes from a human A(H3N2) virus circulating around
2004–2006, and the internal genes from triple-reassortant swine influenza A viruses
(IAVs). To assess population susceptibility to this virus we measured haemagglutination
inhibiting (HI) titres to A/swine/BinhDuong/03-9/2010 and to seasonal A/Perth/16/2009 for
947 sera collected from urban and rural Vietnamese people during 2011–2012. Seroprevalence
(HI ⩾ 40) was high and similar for both viruses, with 62·6% [95% confidence interval (CI)
59·4–65·7] against A/Perth/16/2009 and 54·6% (95% CI 51·4–57·8%) against
A/swine/BinhDuong/03-9/2010, and no significant differences between urban and rural
participants. Children aged <5 years lacked antibodies to the swine origin H3 virus
despite high seroprevalence for A/Perth/16/2009. These results reveal vulnerability to
infection to this contemporary swine IAV in children aged <5 years; however,
cross-reactive immunity in adults would likely limit epidemic emergence potential.</p></div></front><back><div1 type="bibliography"><listBibl><biblStruct><analytic><author><name sortKey="Nelson, Mi" uniqKey="Nelson M">MI Nelson</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Vijaykrishna, D" uniqKey="Vijaykrishna D">D Vijaykrishna</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Campitelli, L" uniqKey="Campitelli L">L Campitelli</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Harder, Tc" uniqKey="Harder T">TC Harder</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Ngo, Lt" uniqKey="Ngo L">LT Ngo</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Dao, Tt" uniqKey="Dao T">TT Dao</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Houser, Kv" uniqKey="Houser K">KV Houser</name></author><author><name sortKey="Katz, Jm" uniqKey="Katz J">JM Katz</name></author><author><name sortKey="Tumpey, Tm" uniqKey="Tumpey T">TM Tumpey</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Skowronski, Dm" uniqKey="Skowronski D">DM Skowronski</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Lessler, J" uniqKey="Lessler J">J Lessler</name></author></analytic></biblStruct><biblStruct><analytic><author><name sortKey="Fonville, Jm" uniqKey="Fonville J">JM Fonville</name></author></analytic></biblStruct></listBibl></div1></back></TEI><pmc article-type="brief-report"><pmc-dir>properties open_access</pmc-dir>
<front><journal-meta><journal-id journal-id-type="nlm-ta">Epidemiol Infect</journal-id><journal-id journal-id-type="iso-abbrev">Epidemiol. Infect</journal-id><journal-id journal-id-type="publisher-id">HYG</journal-id><journal-title-group><journal-title>Epidemiology and Infection</journal-title></journal-title-group><issn pub-type="ppub">0950-2688</issn><issn pub-type="epub">1469-4409</issn><publisher><publisher-name>Cambridge University Press</publisher-name><publisher-loc>Cambridge, UK</publisher-loc></publisher></journal-meta><article-meta><article-id pub-id-type="pmid">25761949</article-id><article-id pub-id-type="pmc">4595856</article-id><article-id pub-id-type="doi">10.1017/S0950268815000187</article-id><article-id pub-id-type="pii">S0950268815000187</article-id><article-id pub-id-type="publisher-id">00018</article-id><article-categories><subj-group subj-group-type="heading"><subject>Short Report</subject></subj-group><subj-group subj-group-type="section"><subject>Influenza</subject></subj-group></article-categories><title-group><article-title>Population susceptibility to a variant swine-origin influenza virus A(H3N2)
in Vietnam, 2011–2012</article-title><alt-title alt-title-type="left-running">L. N. M. Hoa and others</alt-title><alt-title alt-title-type="right-running">Human susceptibility to swine influenza in
Vietnam</alt-title></title-group><contrib-group><contrib contrib-type="author"><name><surname>HOA</surname><given-names>L. N. M.</given-names></name><xref ref-type="aff" rid="aff1">1</xref></contrib><contrib contrib-type="author"><name><surname>BRYANT</surname><given-names>J. E.</given-names></name><xref ref-type="aff" rid="aff1">1</xref><xref ref-type="aff" rid="aff2">2</xref><xref ref-type="corresp" rid="cor1">*</xref></contrib><contrib contrib-type="author"><name><surname>CHOISY</surname><given-names>M.</given-names></name><xref ref-type="aff" rid="aff1">1</xref><xref ref-type="aff" rid="aff3">3</xref></contrib><contrib contrib-type="author"><name><surname>NGUYET</surname><given-names>L. A.</given-names></name><xref ref-type="aff" rid="aff1">1</xref></contrib><contrib contrib-type="author"><name><surname>BAO</surname><given-names>N. T.</given-names></name><xref ref-type="aff" rid="aff1">1</xref></contrib><contrib contrib-type="author"><name><surname>TRANG</surname><given-names>N. H.</given-names></name><xref ref-type="aff" rid="aff1">1</xref></contrib><contrib contrib-type="author"><name><surname>CHUC</surname><given-names>N. T. K.</given-names></name><xref ref-type="aff" rid="aff4">4</xref></contrib><contrib contrib-type="author"><name><surname>TOAN</surname><given-names>T. K.</given-names></name><xref ref-type="aff" rid="aff4">4</xref></contrib><contrib contrib-type="author"><name><surname>SAITO</surname><given-names>T.</given-names></name><xref ref-type="aff" rid="aff5">5</xref></contrib><contrib contrib-type="author"><name><surname>TAKEMAE</surname><given-names>N.</given-names></name><xref ref-type="aff" rid="aff5">5</xref></contrib><contrib contrib-type="author"><name><surname>HORBY</surname><given-names>P.</given-names></name><xref ref-type="aff" rid="aff1">1</xref><xref ref-type="aff" rid="aff2">2</xref></contrib><contrib contrib-type="author"><name><surname>WERTHEIM</surname><given-names>H.</given-names></name><xref ref-type="aff" rid="aff1">1</xref><xref ref-type="aff" rid="aff2">2</xref></contrib><contrib contrib-type="author"><name><surname>FOX</surname><given-names>A.</given-names></name><xref ref-type="aff" rid="aff1">1</xref><xref ref-type="aff" rid="aff6">6</xref></contrib></contrib-group><aff id="aff1"><label>1</label><institution>Oxford University Clinical Research Unit and Wellcome Trust Major Overseas Programme</institution>,<country>Vietnam</country></aff><aff id="aff2"><label>2</label><addr-line>Nuffield Department of Medicine</addr-line>,<institution>Oxford University</institution>,<addr-line>Oxford</addr-line>,<country>UK</country></aff><aff id="aff3"><label>3</label><institution>MIVEGEC</institution> (<addr-line>UMR Universités Montpellier 1 & 2</addr-line>,<addr-line>CNRS 5290</addr-line>,<addr-line>IRD 224</addr-line>),<addr-line>Montpellier</addr-line>,<country>France</country></aff><aff id="aff4"><label>4</label><institution>Hanoi Medical University</institution>,<addr-line>Hanoi</addr-line>,<country>Vietnam</country></aff><aff id="aff5"><label>5</label><addr-line>Influenza and Prion Diseases Research Center</addr-line>,<institution>National Institute of Animal Health</institution>,<addr-line>National Agriculture and Food Research Organization (NARO)</addr-line>,<addr-line>Ibaraki</addr-line>,<country>Japan</country></aff><aff id="aff6"><label>6</label><institution>The University of Melbourne</institution>,<addr-line>Peter Doherty Institute for Infection and Immunity</addr-line>,<addr-line>Department of Microbiology and Immunology</addr-line>,<addr-line>Parkville</addr-line>,<addr-line>Victoria</addr-line>,<country>Australia</country></aff><author-notes><corresp id="cor1"><label>*</label><addr-line>Author for correspondence: J. E.
Bryant</addr-line>, <institution>Oxford University Clinical Research Unit
Vietnam</institution>, <addr-line>National Hospital of Tropical Diseases</addr-line>,
<addr-line>78 Giai Phong Street</addr-line>, <addr-line>Dong Da District</addr-line>,
<addr-line>Hanoi</addr-line>, <country>Vietnam</country> (Email: <email>jbryant@oucru.org</email>)</corresp></author-notes><pub-date pub-type="ppub"><month>10</month><year>2015</year></pub-date><pub-date pub-type="epub"><day>12</day><month>3</month><year>2015</year></pub-date><volume>143</volume><issue>14</issue><fpage>2959</fpage><lpage>2964</lpage><history><date date-type="received"><day>15</day><month>8</month><year>2014</year></date><date date-type="rev-recd"><day>16</day><month>1</month><year>2015</year></date><date date-type="accepted"><day>21</day><month>1</month><year>2015</year></date></history><permissions><copyright-statement>© Cambridge University Press 2015</copyright-statement><copyright-year>2015</copyright-year><copyright-holder>Cambridge University Press</copyright-holder><license license-type="open-access" xlink:href="http://creativecommons.org/licenses/by/3.0/"><license-p><pmc-comment>CREATIVE COMMONS</pmc-comment>This is an Open Access article, distributed under the terms of the Creative
Commons Attribution licence (<ext-link ext-link-type="uri" xlink:href="http://creativecommons.org/licenses/by/3.0/">http://creativecommons.org/licenses/by/3.0/</ext-link>), which permits
unrestricted re-use, distribution, and reproduction in any medium, provided the original
work is properly cited.</license-p></license></permissions><self-uri xlink:title="pdf" xlink:href="S0950268815000187a.pdf"></self-uri><abstract abstract-type="normal"><title>SUMMARY</title><p>A reassortant swine-origin A(H3N2) virus (A/swine/BinhDuong/03-9/2010) was detected
through swine surveillance programmes in southern Vietnam in 2010. This virus contains
haemagglutinin and neuraminidase genes from a human A(H3N2) virus circulating around
2004–2006, and the internal genes from triple-reassortant swine influenza A viruses
(IAVs). To assess population susceptibility to this virus we measured haemagglutination
inhibiting (HI) titres to A/swine/BinhDuong/03-9/2010 and to seasonal A/Perth/16/2009 for
947 sera collected from urban and rural Vietnamese people during 2011–2012. Seroprevalence
(HI ⩾ 40) was high and similar for both viruses, with 62·6% [95% confidence interval (CI)
59·4–65·7] against A/Perth/16/2009 and 54·6% (95% CI 51·4–57·8%) against
A/swine/BinhDuong/03-9/2010, and no significant differences between urban and rural
participants. Children aged <5 years lacked antibodies to the swine origin H3 virus
despite high seroprevalence for A/Perth/16/2009. These results reveal vulnerability to
infection to this contemporary swine IAV in children aged <5 years; however,
cross-reactive immunity in adults would likely limit epidemic emergence potential.</p></abstract><kwd-group><title>Key words</title><kwd>Influenza A</kwd><kwd>virology (human) and epidemiology</kwd><kwd>zoonoses</kwd></kwd-group><counts><fig-count count="2"></fig-count><table-count count="2"></table-count><ref-count count="10"></ref-count><page-count count="6"></page-count></counts></article-meta></front><body><p>Studies of the human–swine interface for influenza A virus (IAV) transmission are important
to understand the complex multi-host ecology of influenza, and to assess potential disease
risks. IAVs currently circulating in global swine populations comprise three subtypes (H1N1,
H1N2, H3N2), and are similar to those circulating in humans (H1N1 and H3N2). Recent
large-scale assessments of IAV diversity in swine have highlighted the frequency of
human-to-swine transmission (so-called ‘reverse zoonoses’), which appears to be far more
common than swine-to-human transmission [<xref rid="ref1" ref-type="bibr">1</xref>]. In
particular, human H3N2 viruses have entered swine populations on numerous occasions, beginning
in 1980s in Europe, in the 1990s in North America, and later observed in many countries and
all continents [<xref rid="ref1" ref-type="bibr">1</xref>–<xref rid="ref4" ref-type="bibr">4</xref>]. Typically, onward transmission of the human-origin viruses in swine has led to
subsequent reassortment, and divergence into lineages with mixed genome constellations of
swine, human, and avian origin [<xref rid="ref2" ref-type="bibr">2</xref>]. Importantly, the
process of antigenic drift that is characteristic of H3N2 evolution in human populations is
believed to operate on a slower time scale in swine [<xref rid="ref1" ref-type="bibr">1</xref>]. Thus, swine IAV with envelope haemagglutinin (HA) and neuraminidase (NA) proteins
derived from ‘old’ human seasonal strains – i.e. viruses that have since disappeared from
human circulation – may continue to circulate in swine for many years with relatively few
antigenic changes, and disease risks associated with these variant viruses remain poorly
understood.</p><p>In recent years, surveillance of swine influenza viruses in Vietnam has received much
attention, due to the perception of Vietnam as a ‘hotspot’ for viral emergence and the
inherent dangers of co-circulation of highly pathogenic avian viruses (H5N1) in areas with
dense poultry and pig populations. The first detection of human-origin influenza viruses in
swine in Vietnam was a reassortant A(H3N2) (A/swine/BinhDuong/03–9/2010, and hereafter
referred to as Sw/VN10) found in 2010 from a commercial pig farm located in the southeast
region of Vietnam, to the north of Ho Chi Minh City [<xref rid="ref5" ref-type="bibr">5</xref>]. Genetic analysis revealed that the six Binh Duong H3N2 swine viruses isolated at
one time from the same farm had HA similar to human seasonal viruses circulating between 2004
and 2006 (<xref ref-type="fig" rid="fig01">Fig. 1</xref>) [<xref rid="ref5" ref-type="bibr">5</xref>]. The estimated time to most recent common ancestor of the Vietnamese swine-origin
Binh Duong H3 cluster suggested human-to-pig transmission in late 2004 (95% highest posterior
density value, 23 October 2004 to 16 December 2004). Antigenically, Sw/VN10 exhibited a
profile that was distinguishable from contemporary human H3 viruses, but showed some level of
cross-reactivity with antisera of human seasonal vaccines and recent cluster variants (<xref ref-type="table" rid="tab01">Table 1</xref>, e.g. 1:320 to A/Wyoming/03 and 1:140 to
A/Wuhan/95). Although swine surveillance data from Vietnam is not available prior to 2010,
Sw/VN10-like viruses were isolated each year from southern Vietnamese swine from 2010 to 2014,
suggesting that it is has been one of the dominant contemporary swine IAVs circulating in
Vietnam, at least for the past 4 years. The human seasonal H3N2 viruses circulating in the
region during 2007–2008 were A/H3N2/Brisbane/10/2007-like, with a cluster transition to
A/Perth/16/2009 (hereafter referred to as Pe09) in 2009 that has persisted until at least
mid-2013. <fig id="fig01" orientation="portrait" position="float"><label>Fig. 1.</label><caption><p>Bayesian Markov chain Monte Carlo phylogenetic tree based on nucleotide sequences of H3
HA genes of selected H3N2 viruses. The analysis was performed in Beast package v. 1.7.4
(<uri xlink:type="simple" xlink:href="http://beast.bio.ed.ac.uk/">http://beast.bio.ed.ac.uk/</uri>) using 164 HA sequences (1701 bp) of H3N2 viruses
isolated from 2000 to 2013. Node bars show the estimated divergent period to most recent
common ancestor (95% highest posterior density).</p></caption><graphic xlink:href="S0950268815000187_fig1"></graphic></fig><table-wrap id="tab01" orientation="portrait" position="float"><label>Table 1.</label><caption><p>Antigenic characterization of Sw/VN10 by haemagglutination inhibition using reference
chicken antisera to Vietnamese swine and avian H3 viruses, and ferret antisera to human
H3N2 vaccine strains</p></caption><alternatives><graphic xlink:href="S0950268815000187_tab1"></graphic><table frame="hsides" rules="groups"><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><col align="left" width="1*" span="1"></col><thead><tr><th align="left" rowspan="5" valign="bottom" colspan="1">Virus</th><th colspan="8" align="left" rowspan="1" valign="bottom">HI titres with</th></tr><tr><th colspan="2" align="left" rowspan="1">Hyperimmune chicken antisera against</th><th colspan="6" align="left" rowspan="1"></th></tr><tr><th align="left" rowspan="2" valign="bottom" colspan="1">Vietnamese swine IAV</th><th align="left" rowspan="2" valign="bottom" colspan="1">AIV</th><th colspan="6" align="left" rowspan="1">Infected ferret sera</th></tr><tr><th colspan="6" align="left" rowspan="1">Human vaccine strains</th></tr><tr><th align="left" rowspan="1" colspan="1">BD014/10</th><th align="left" rowspan="1" colspan="1">Bud77</th><th align="left" rowspan="1" colspan="1">Wuh95</th><th align="left" rowspan="1" colspan="1">Syd97</th><th align="left" rowspan="1" colspan="1">Wyo03</th><th align="left" rowspan="1" colspan="1">NY04</th><th align="left" rowspan="1" colspan="1">Uru07</th><th align="left" rowspan="1" colspan="1">Vic11</th></tr></thead><tbody><tr><td rowspan="1" colspan="1">Human-like SIVs</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1">A/swine/Binh Duong/03-9/2010 (H3N2)</td><td rowspan="1" colspan="1">1280</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">160</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">320</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/swine/Binh Duong/03-14/2010 (H3N2)</td><td rowspan="1" colspan="1"><bold>2560</bold></td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">160</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">320</td><td rowspan="1" colspan="1">80</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">40</td></tr><tr><td rowspan="1" colspan="1">Seasonal human virus</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1">A/Port Chalmer/1/1973 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/Victoria/3/1975 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/Bangkok/1/1979 (H3N2)</td><td rowspan="1" colspan="1">80</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/Wuhan/359/1995 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><bold>320</bold></td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/Sydney/5/1997 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">640</td><td rowspan="1" colspan="1"><bold>1280</bold></td><td rowspan="1" colspan="1">640</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/Wyoming/3/2003 (H3N2)</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">320</td><td rowspan="1" colspan="1">320</td><td rowspan="1" colspan="1"><bold>2560</bold></td><td rowspan="1" colspan="1">2560</td><td rowspan="1" colspan="1">1280</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/New York/55/2004 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">1280</td><td rowspan="1" colspan="1"><bold>5120</bold></td><td rowspan="1" colspan="1">1280</td><td rowspan="1" colspan="1"><40</td></tr><tr><td rowspan="1" colspan="1">A/Uruguay/716/2007 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">160</td><td rowspan="1" colspan="1">1280</td><td rowspan="1" colspan="1"><bold>2560</bold></td><td rowspan="1" colspan="1">40</td></tr><tr><td rowspan="1" colspan="1">A/Perth/16/2009 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">40</td><td rowspan="1" colspan="1">80</td><td rowspan="1" colspan="1">320</td></tr><tr><td rowspan="1" colspan="1">A/Victoria/361/2011 (H3N2)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1">80</td><td rowspan="1" colspan="1">160</td><td rowspan="1" colspan="1">320</td><td rowspan="1" colspan="1"><bold>1280</bold></td></tr><tr><td rowspan="1" colspan="1">Avian influenza virus</td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td><td rowspan="1" colspan="1"></td></tr><tr><td rowspan="1" colspan="1">A/budgerigar/Aichi/1/1977 (H3N8)</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><bold>10240</bold></td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td><td rowspan="1" colspan="1"><40</td></tr></tbody></table></alternatives><table-wrap-foot><fn><p>IAV, Influenza A virus; AIV, avian influenza virus; SIV, swine influenza virus.</p></fn><fn><p>Homologous titres are indicated in bold.</p></fn></table-wrap-foot></table-wrap></p><p>In this study, we aimed to investigate the susceptibility of the Vietnamese human population
to Sw/VN10 compared to contemporary circulating human H3N2 (Pe09) using serum collections from
a previous comparative study of urban and rural residential populations [<xref rid="ref6" ref-type="bibr">6</xref>]. A total of 943 participants were enrolled: 495 urban residents from
Hanoi city and 448 rural residents of Ba Vi district, located 60 km southwest of Hanoi. About
40% of participants were aged <20 years (mean age 33 years, range 2–91 years); sampling
was age-stratified and designed to over-represent children aged <20 years, hence the
age distribution of samples did not reflect population structure. A single serum sample was
obtained from each of the rural participants (between March and May 2012), whereas paired
serum collections were obtained from the urban participants (278 pairs, collected in October
2011 and February/March 2012). Prior to performing HI analysis, the reactivity of the two
antigens using guinea pig and turkey erythrocytes was evaluated in parallel. Both the Pe09 and
Sw/VN10 antigens exhibited twofold higher HA titres when using turkey <italic>vs</italic>.
guinea pig, and the agglutination phenotype was considerably more stable using turkey
erythrocytes. Hence, HI assays were performed using standard methods with 0·5% turkey
erythrocytes and a starting dilution of 1:10. For single sera analysis, ‘seropositive’ was
defined as a titre of ⩾40; for paired sera analysis, ‘seroconversion’ was defined as a
fourfold or greater rise in HI titre, with a second titre of at least 40.</p><p>The overall seroprevalence of titres ⩾40 against Pe09 and Sw/VN10 was 62·6% [95% confidence
interval (CI) 59·4–65·7] and 54·6% (95% CI 51·4–57·8), respectively (<xref ref-type="table" rid="tab02">Table 2</xref>). Correlation analysis of Pe09 and Sw/VN10 HI titres by Spearman
coefficient was 0·426 (<italic>r</italic><sub><italic>s</italic></sub> = 82316028, <italic>P</italic> < 0·001), consistent with significant
cross-reactivity to the two antigens. In children aged <5 years, only 9/70 (12·9%) were
seropositive to Sw/VN10, whereas 48/70 (68·6%) were seropositive to Pe09; these proportions
were highly significantly different (<italic>χ</italic><sup>2</sup> = 42·7, <sc>d.f</sc>. = 1,
<italic>P</italic> < 0·001). <xref ref-type="fig" rid="fig02">Figure 2</xref> shows
the polynomial logistic regressions of seroprevalence as a function of age for Pe09 and
Sw/VN10. Titres to Pe09 were highest in children aged <15 years, whereas highest titres
to Sw/VN10 were in young adults aged 15–25 years, with very low titres observed in children
aged <10 years, and a trough in titres for people aged 40–60 years. We propose that
this pattern of higher reactivity in young adults reflects the influenza strains that
individuals may have first experienced as young children, i.e. A/Wuhan/95 and A/Wyoming/03.
Sex ratios were not significantly different between the two sites
(<italic>χ</italic><sup>2</sup> = 0·0105, <sc>d.f.</sc> = 1, <italic>P</italic> = 0·9182).
However, mean ages were higher in Dong Da (34·8 years) than in Ba Vi (31·2 years)
(<italic>t</italic> = 2·335, <sc>d.f.</sc> = 936·809, <italic>P</italic> = 0·020). Despite
this, the age profile of HI titres was not different between the two sites, neither for Pe09
(likelihood ratio test: <italic>χ</italic><sup>2</sup> = 0·202, <sc>d.f.</sc> = 1,
<italic>P</italic> = 0·6533), nor for Sw/VN10 (likelihood ratio test:
<italic>χ</italic><sup>2</sup> = 1·266, <sc>d.f.</sc> = 1, <italic>P</italic> = 0·2606)
(Supplementary Fig. S1). <fig id="fig02" orientation="portrait" position="float"><label>Fig. 2.</label><caption><p>Percentage of individuals with an HI titre ⩾40 (%) against Pe09 (blue) and Sw/VN10
(red). Dots and vertical bars show the mean seroprevalences and their 95% confidence
intervals for the nine age groups defined by the thin vertical grey lines. The limits of
these age groups were chosen so that they all contain approximately the same number of
samples. The curves show the models of the polynomial logistic regressions, up until
degree 5 (degree 6 being non-significantly different from 0). The coloured area shows
the 95% confidence intervals of the models' predictions.</p></caption><graphic xlink:href="S0950268815000187_fig2"></graphic></fig><table-wrap id="tab02" orientation="portrait" position="float"><label>Table 2.</label><caption><p>Demographic characteristics of the study participants and haemagglutinin inhibition
(HI) antibody titres by age and influenza strain. Seropositive defined as titre ⩾40;
seroconversion defined by fourfold or greater rise in HI titre, with a second titre at
least 1:40</p></caption><alternatives><graphic xlink:href="S0950268815000187_tab2"></graphic><table frame="hsides" rules="groups"><col align="left" width="1*" span="1"></col><col align="char" char="." width="1*" span="1"></col><col align="char" char="." width="1*" span="1"></col><col align="char" char="·" width="1*" span="1"></col><col align="char" char="·" width="1*" span="1"></col><col align="char" char="." width="1*" span="1"></col><col align="char" char="." width="1*" span="1"></col><col align="char" char="(" width="1*" span="1"></col><col align="char" char="(" width="1*" span="1"></col><col align="char" char="–" width="1*" span="1"></col><col align="char" char="–" width="1*" span="1"></col><col align="char" char="(" width="1*" span="1"></col><col align="char" char="(" width="1*" span="1"></col><thead><tr><th align="left" rowspan="2" valign="bottom" colspan="1">Age group, yr</th><th align="left" rowspan="2" valign="bottom" colspan="1">Total no. sera</th><th align="left" rowspan="2" valign="bottom" colspan="1">No. pair sera</th><th align="left" rowspan="2" valign="bottom" colspan="1">Mean age, yr</th><th align="left" rowspan="2" valign="bottom" colspan="1">Female sex (%)</th><th align="left" rowspan="2" valign="bottom" colspan="1">No. Dong Da (urban)</th><th align="left" rowspan="2" valign="bottom" colspan="1">No. Ba Vi (rural)</th><th colspan="2" align="left" rowspan="1">Seropositives (%)</th><th colspan="2" align="left" rowspan="1">GMT (95% CI)</th><th colspan="2" align="left" rowspan="1">Seroconversions (%)</th></tr><tr><th align="left" rowspan="1" colspan="1" valign="bottom">A/H3/Pe09</th><th align="left" rowspan="1" colspan="1" valign="bottom">A/Sw/VN10</th><th align="left" rowspan="1" colspan="1" valign="bottom">A/H3/Pe09</th><th align="left" rowspan="1" colspan="1" valign="bottom">A/Sw/VN10</th><th align="left" rowspan="1" colspan="1" valign="bottom">A/H3/Pe09</th><th align="left" rowspan="1" colspan="1" valign="bottom">A/Sw/VN10</th></tr></thead><tbody><tr><td rowspan="1" colspan="1"><5</td><td rowspan="1" colspan="1">70</td><td rowspan="1" colspan="1">13</td><td rowspan="1" colspan="1">3·7</td><td rowspan="1" colspan="1">47·1</td><td rowspan="1" colspan="1">31</td><td rowspan="1" colspan="1">39</td><td rowspan="1" colspan="1">48 (68·6%)</td><td rowspan="1" colspan="1">9 (12·8%)</td><td rowspan="1" colspan="1">5·8 (5·3–6·2)</td><td rowspan="1" colspan="1">3·2 (2·9–3·4)</td><td rowspan="1" colspan="1">1 (7·7%)</td><td rowspan="1" colspan="1">0 (0%)</td></tr><tr><td rowspan="1" colspan="1">5–9</td><td rowspan="1" colspan="1">121</td><td rowspan="1" colspan="1">31</td><td rowspan="1" colspan="1">7·5</td><td rowspan="1" colspan="1">45·5</td><td rowspan="1" colspan="1">51</td><td rowspan="1" colspan="1">70</td><td rowspan="1" colspan="1">103 (85·1%)</td><td rowspan="1" colspan="1">52 (43%)</td><td rowspan="1" colspan="1">6·2 (5·9–6·5)</td><td rowspan="1" colspan="1">4·5 (4·2–4·8)</td><td rowspan="1" colspan="1">2 (6·4%)</td><td rowspan="1" colspan="1">1 (3·2%)</td></tr><tr><td rowspan="1" colspan="1">10–14</td><td rowspan="1" colspan="1">126</td><td rowspan="1" colspan="1">32</td><td rowspan="1" colspan="1">12·3</td><td rowspan="1" colspan="1">52·4</td><td rowspan="1" colspan="1">62</td><td rowspan="1" colspan="1">64</td><td rowspan="1" colspan="1">98 (77·8%)</td><td rowspan="1" colspan="1">105 (83·3%)</td><td rowspan="1" colspan="1">5·7 (5·4–5·9)</td><td rowspan="1" colspan="1">6·2 (5·9–6·4)</td><td rowspan="1" colspan="1">4 (12·5%)</td><td rowspan="1" colspan="1">1 (3·1%)</td></tr><tr><td rowspan="1" colspan="1">15–19</td><td rowspan="1" colspan="1">83</td><td rowspan="1" colspan="1">29</td><td rowspan="1" colspan="1">17·6</td><td rowspan="1" colspan="1">54·2</td><td rowspan="1" colspan="1">53</td><td rowspan="1" colspan="1">30</td><td rowspan="1" colspan="1">53 (63·9%)</td><td rowspan="1" colspan="1">72 (86·7%)</td><td rowspan="1" colspan="1">5·3 (4·9–5·6)</td><td rowspan="1" colspan="1">6·6 (6·2–7·0)</td><td rowspan="1" colspan="1">1 (3·4%)</td><td rowspan="1" colspan="1">1 (3·4%)</td></tr><tr><td rowspan="1" colspan="1">20–29</td><td rowspan="1" colspan="1">83</td><td rowspan="1" colspan="1">19</td><td rowspan="1" colspan="1">25·4</td><td rowspan="1" colspan="1">55·4</td><td rowspan="1" colspan="1">48</td><td rowspan="1" colspan="1">35</td><td rowspan="1" colspan="1">52 (62·7%)</td><td rowspan="1" colspan="1">62 (74·7%)</td><td rowspan="1" colspan="1">5·1 (4·8–5·5)</td><td rowspan="1" colspan="1">6·1 (5·7–6·4)</td><td rowspan="1" colspan="1">1 (5·3%)</td><td rowspan="1" colspan="1">0 (0%)</td></tr><tr><td rowspan="1" colspan="1">30–39</td><td rowspan="1" colspan="1">114</td><td rowspan="1" colspan="1">28</td><td rowspan="1" colspan="1">35·2</td><td rowspan="1" colspan="1">64</td><td rowspan="1" colspan="1">60</td><td rowspan="1" colspan="1">54</td><td rowspan="1" colspan="1">60 (52·6%)</td><td rowspan="1" colspan="1">61 (53·5%)</td><td rowspan="1" colspan="1">4·7 (4·5–5·0)</td><td rowspan="1" colspan="1">4·8 (4·6–5·1)</td><td rowspan="1" colspan="1">3 (10·7%)</td><td rowspan="1" colspan="1">4 (14·3%)</td></tr><tr><td rowspan="1" colspan="1">40–49</td><td rowspan="1" colspan="1">86</td><td rowspan="1" colspan="1">24</td><td rowspan="1" colspan="1">44·7</td><td rowspan="1" colspan="1">56·9</td><td rowspan="1" colspan="1">37</td><td rowspan="1" colspan="1">49</td><td rowspan="1" colspan="1">41 (47·7%)</td><td rowspan="1" colspan="1">31 (36%)</td><td rowspan="1" colspan="1">4·5 (4·2–4·8)</td><td rowspan="1" colspan="1">4·3 (3·9–4·6)</td><td rowspan="1" colspan="1">2 (8·3%)</td><td rowspan="1" colspan="1">0 (0%)</td></tr><tr><td rowspan="1" colspan="1">50–59</td><td rowspan="1" colspan="1">93</td><td rowspan="1" colspan="1">30</td><td rowspan="1" colspan="1">55·1</td><td rowspan="1" colspan="1">63·4</td><td rowspan="1" colspan="1">51</td><td rowspan="1" colspan="1">42</td><td rowspan="1" colspan="1">48 (51·6%)</td><td rowspan="1" colspan="1">41 (44·1%)</td><td rowspan="1" colspan="1">4·6 (4·4–4·9)</td><td rowspan="1" colspan="1">4·5 (4·2–4·8)</td><td rowspan="1" colspan="1">3 (10%)</td><td rowspan="1" colspan="1">1 (3·3%)</td></tr><tr><td rowspan="1" colspan="1">60–69</td><td rowspan="1" colspan="1">84</td><td rowspan="1" colspan="1">39</td><td rowspan="1" colspan="1">65·4</td><td rowspan="1" colspan="1">48·8</td><td rowspan="1" colspan="1">51</td><td rowspan="1" colspan="1">33</td><td rowspan="1" colspan="1">42 (50%)</td><td rowspan="1" colspan="1">41 (48%)</td><td rowspan="1" colspan="1">4·6 (4·3–4·9)</td><td rowspan="1" colspan="1">4·8 (4·4–5·2)</td><td rowspan="1" colspan="1">5 (12·8%)</td><td rowspan="1" colspan="1">3 (7·7%)</td></tr><tr><td rowspan="1" colspan="1">>69</td><td rowspan="1" colspan="1">83</td><td rowspan="1" colspan="1">33</td><td rowspan="1" colspan="1">77·2</td><td rowspan="1" colspan="1">55·4</td><td rowspan="1" colspan="1">51</td><td rowspan="1" colspan="1">32</td><td rowspan="1" colspan="1">45 (54·2%)</td><td rowspan="1" colspan="1">41 (49·4%)</td><td rowspan="1" colspan="1">4·9 (4·5–5·3)</td><td rowspan="1" colspan="1">4·9 (4·4–5·3)</td><td rowspan="1" colspan="1">9 (27·2%)</td><td rowspan="1" colspan="1">3 (10%)</td></tr><tr><td rowspan="1" colspan="1">Overall</td><td rowspan="1" colspan="1">943</td><td rowspan="1" colspan="1">278</td><td rowspan="1" colspan="1">33·1</td><td rowspan="1" colspan="1">54·4</td><td rowspan="1" colspan="1">495</td><td rowspan="1" colspan="1">448</td><td rowspan="1" colspan="1">590 (62·6%)</td><td rowspan="1" colspan="1">515 (54·6%)</td><td rowspan="1" colspan="1">5·2 (5·1–5·3)</td><td rowspan="1" colspan="1">5·0 (4·9–5·2)</td><td rowspan="1" colspan="1">31 (11·1%)</td><td rowspan="1" colspan="1">14 (5·03%)</td></tr></tbody></table></alternatives><table-wrap-foot><fn><p>GMT, Geometric mean titre of log<sub>2</sub>-transformed HI titre; CI, confidence
interval.</p></fn></table-wrap-foot></table-wrap></p><p>Analysis of paired sera from 278 urban participants found 11·1% (31/278) with seroconversions
to Pe09 and 5·0% (14/278) with seroconversions to Sw/VN10, of whom five were dual
seroconversions to both Sw/WN10 and Pe09. Dual seroconverters were exclusively found in the
30–39 and >69 years age groups (Supplementary Table S1). Influenza vaccination status
of participants indicated that only 51 had been vaccinated against influenza within the past
year, 842 had not been vaccinated within the past year, and 43 did not know their status. In
people that reported recent vaccination (for which the H3 vaccine virus would have been Pe09),
HI titres against Pe09 were higher compared to those who were not vaccinated, and there were
no seroconversions in vaccinated people compared to 11·4% (95% CI 9·4–13·8) seroconversions to
Pe09 in non-vaccinated individuals (Supplementary Fig. S2). Titres to Sw/VN10 were similar in
both vaccinated and non-vaccinated individuals [<xref rid="ref7" ref-type="bibr">7</xref>,
<xref rid="ref8" ref-type="bibr">8</xref>].</p><p>In summary, we found that more than 50% of the Vietnamese population had HI titres ⩾40 to
Sw/VN10; however, the variable age profile of HI reactivity suggested significant differences
in susceptibility to infection, with the greatest vulnerability in children aged <5
years. In previous behavioural surveys conducted within the same populations, rural
inhabitants of Ba Vi reported significantly more contact with swine than urban residents (40%
of rural residents reported direct contact with pigs on ‘most days’ whereas 95% of urban
residents ‘never’ had direct contact with pigs) (Supplementary Fig. S3). Given that both rural
and urban cohorts exhibited similar overall prevalence to the two antigens, and given the
distinct age profiles of immunity, it seems highly unlikely that the observed titres to
Sw/VN10 reflect exposure to swine viruses. Rather, we infer that the profile of reactivity to
Sw/VN10 observed in adults is consistent with their previous exposures to seasonal H3 strains
that were antigenically similar to Sw/VN10. This is compatible with previous studies showing
persistent titres to older antigenic clusters [<xref rid="ref9" ref-type="bibr">9</xref>,
<xref rid="ref10" ref-type="bibr">10</xref>], and also underscores the need for
comprehensive testing of antibody response profiles across multiple antigenic clusters when
studying zoonotic risks. Similarly, the relatively low seroprevalence of children aged
<5 years to Sw/VN10 most likely reflects lack of infection with ‘old’ human seasonal H3
variants, as well as lack of infection with Sw/VN10-like swine IAVs.</p><p>Our findings highlight the susceptibility of children to infection with reassortant
swine-human viruses whose HA antigens are divergent from human H3 viruses circulating during
their lifetimes. With increasing information about regional and global swine IAV circulation,
and more swine IAV isolates being generated by national surveillance projects, it may become
feasible to use population immunity data and new computational approaches to predict which
swine IAVs pose the greatest threat of zoonotic emergence. However, our findings reveal the
significant challenge of using seroepidemiological approaches to assess swine-to-human
transmission events, and the need for more comprehensive assessments of population immunity
profiles to multiple current and past H3 viruses of human and swine origin. Given that
multiple different IAVs with swine-human hybrid genomes continue to circulate in pig
populations, the epidemiological relevance of persistent ‘old’ (‘extinct’) human influenza
virus variants in swine populations merits further research.</p></body><back><ack id="ack"><title>ACKNOWLEDGEMENTS</title><p>M.C. is supported by the ‘Biodiversity and Infectious Diseases in Southeast Asia’ GDRI.
T.S. and N.T. are supported by a programme of the Japan Initiative for Global Research
Network on Infectious Diseases (J-GRID) of the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT) of Japan. The authors thank Dr Sabrina L. Swenson, National
Veterinary Services Laboratories, Ames, IA, USA, for providing
A/swine/Indiana/A00968373/2012 (H3N2v). Funding was obtained from the Wellcome Trust of
Great Britain VIZIONS project (WT/093724).</p></ack><sec sec-type="supplementary-material" id="sec1"><title>Supplementary material</title><supplementary-material content-type="local-data" id="S0950268815000187sup001"><p>For supplementary material accompanying this paper visit http://dx.doi.org/10.1017/S0950268815000187.</p><media xlink:href="S0950268815000187sup001.docx" mimetype="DOCX" mime-subtype="openxmlformats-officedocument.wordprocessingml.document" orientation="portrait" id="d35e1255" position="anchor"><caption><p>click here to view supplementary material</p></caption></media></supplementary-material></sec><notes notes-type="other" id="nts2"><title>DECLARATION OF INTEREST</title><p>None.</p></notes><ref-list id="reflist1"><title>REFERENCES</title><ref id="ref1"><label>1.</label><mixed-citation publication-type="journal"><name><surname>Nelson</surname><given-names>MI</given-names></name>, <etal></etal><article-title>Introductions and evolution of human-origin seasonal influenza A viruses in
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