Age-related prevalence of cross-reactive antibodies against influenza A(H3N2) variant virus, Germany, 2003 to 2010.

To estimate susceptibility to the swine-origin influenza A(H3N2) variant virus (A(H3N2) v) in the German population, we investigated cross-reactive antibodies against this virus and factors associated with seroprotective titre using sera from representative health examination surveys of children and adolescents (n = 815, 2003-06) and adults (n = 600, 200810). Antibodies were assessed by haemagglutination inhibition assay (HI); in our study an HI titre >= 40 was defined as seroprotective. We investigated associated factors by multivariable logistic regression. Overall, 41% (95% confidence interval (CI): 37-45) of children and adolescents and 39% (95% CI: 34-44) of adults had seroprotective titres. The proportion of people with seroprotective titre was lowest among children younger than 10 years (15%; 95% CI: 7-30) and highest among adults aged 18 to 29 years (59%; 95% CI: 49-67). Prior influenza vaccination was associated with higher odds of having seroprotective titre (odds ratio (OR) for children and adolescents: 3.4; 95% CI: 1.8-6.5; OR for adults: 2.4; 95% CI: 1.7-3.4). Young children showed the highest and young adults the lowest susceptibility to the A(H3N2) v virus. Our results suggest that initial exposure to circulating seasonal influenza viruses may predict long-term cross-reactivity that may be enhanced by seasonal influenza vaccination.