Population susceptibility to North American and Eurasian swine influenza viruses in England, at three time points between 2004 and 2011.

Age-stratified sera collected in 2004, 2008 and 2010 in England were evaluated for antibody to swine influenza A(H3N2) and A(H1N1) viruses from the United States or Europe as a measure of population susceptibility to the emergence of novel viruses. Children under 11 years of age had little or no measurable antibody to recent swine H3N2 viruses despite their high levels of antibody to recent H3N2 seasonal human strains. Adolescents and young adults (born 1968–1999) had higher antibody levels to swine H3N2 viruses. Antibody levels to swine H3N2 influenza show little correlation with exposure to recent seasonal H3N2 (A/Perth/16/2009) strains, but with antibody to older H3N2 strains represented by A/Wuhan/359/1995. Children had the highest seropositivity to influenza A(H1N1)pdm09 virus, and young adults had the lowest antibody levels to A/Perth/16/2009. No age group showed substantial antibody levels to A/Aragon/RR3218/2008, a European swine H1N1 virus belonging to the Eurasian lineage. After vaccination with contemporary trivalent vaccine we observed evidence of boosted reactivity to swine H3N2 viruses in children and adults, while only a limited boosting effect on antibody levels to A/Aragon/RR3218/2008 was observed in both groups. Overall, our results suggest that different vaccination strategies may be necessary according to age if swine viruses emerge as a significant pandemic threat.