1www.eurosurveillance.org Research articles Influenza A(H1N1)pdm09 antibodies after pandemic

The 2009 influenza pandemic has introduced the new re-assorted influenza A(H1N1)pdm09 virus which recir-culated during the 2010/11 influenza season. Before that season, it was possible to acquire protective immunity either by pandemic or seasonal influenza vaccination against influenza A(H1N1)pdm09 or by nat-ural infection. To obtain data on vaccination coverage and antibody levels in a reference population and to calculate whether or not the herd immunity threshold (HIT, calculated as 33 % given an R0 of 1.5) was reached at the beginning of the 2010/11 season we performed a seroprevalence study in November 2010 in Hamburg, Germany. Antibody titres were assessed applying a haemagglutination inhibition test. Vaccination cov-erage was very low: 14 % for pandemic and 11 % fo