Antibodies against 1940s era a/H1N1 influenza strains a/Weiss/43 and a/FM/1/47 and heterotypic responses after seasonal vaccination of an elderly Spanish population

Abstract

Background and methods Elderly people have experienced several influenza natural infections and seasonal vaccinations during their lives. The aim of this work was to evaluate in an elderly Spanish population the presence of antibodies (Abs) against some 1940s era A/H1N1 influenza viruses and some new influenza viruses. We also evaluated the homologous and heterotypic responses after seasonal influenza vaccination. We collected pre- and post-vaccination serum samples from 174 elderly people (≥65 years) who were vaccinated with seasonal influenza vaccines during the 2006–2007, 2008–2009, 2009–2010, and 2010–2011 northern hemisphere influenza campaigns. The presence of Abs against the 1940s era A/Weiss/43 and A/FM/1/47 strains of the A/H1N1 influenza virus was evaluated by using hemagglutination inhibition assays. Results Pre-vaccination Abs against the A/Weiss/43 and A/FM/1/47 strains were present at protective titres (≥1/40) in 43.7% and 20.1% of the study population respectively. Seasonal influenza vaccination induced heterotypic seroconversion against A/Weiss/43 in 16.1% of the individuals and against A/FM/1/47 in 13.2% of the individuals. The seroprotection rate for the study population after seasonal vaccination was 63.2% against A/Weiss/43 and 31.0% against A/FM/1/47. The heterotypic response did not satisfy the European Medicament Agency criteria for people aged ≥60 years. Conclusions A moderate percentage of elderly people had Abs against the 1940s era A/Weiss/43 and A/FM/1/47 strains of the A/H1N1 influenza subtype. Seasonal influenza vaccination induced a low but significant heterotypic response against both 1940s era influenza strains, reaching a high seroprotection rate for the A/Weiss/43 strain. Seasonal influenza vaccination can increase, within certain limitations, the Abs titres against old influenza strains not included in the composition of the vaccine itself.