We looked for evidence of antibodies to the 2009 influenza A/H1N1 pandemic virus in panels of sera from individuals living in metropolitan France, obtained either before, during or after the epidemic, using standard haemagglutination inhibition and microneutralization tests. The difference between seroprevalence values measured in post- and pre-epidemic panels was used as an estimate of seroconversion rate in different age groups (23.4% (0-24 years, age-group 0); 16.5% (25-34); 7.9% (35-44); 7.2% (45-54); 1.6% (55-64); and 3.1% (>65)), confirming that the distribution of cases in different age groups was similar to that of the seasonal H1N1 virus. During the pre-pandemic period low-titre cross-reactive antibodies were present in a large proportion of the population (presumably acquired against seasonal H1N1) whereas cross-reactive antibodies were detected in individuals over the age of 65 years with significantly higher prevalence and serological titres (presumably acquired previously against Spanish flu-related H1N1 strains). Clinical data and analysis of post-pandemic seroprevalence showed that few of these latter patients were infected by the influenza virus during the epidemic. In contrast, the majority of both clinical cases and seroconversions were recorded in the 0-24 age group and a global inverse relationship between prevalence of antibodies to pH1N1 in the pre-pandemic period and rate of seroconversion was observed amongst age groups. Our results emphasize the complex relationships involved in antigenic reactivity to pandemic and seasonal H1N1 viral antigens; hence the difficulty in distinguishing between low-titre specific and cross-reactive antibodies, establishing precise seroprevalence numbers and fully understanding the relationship between previous immunity to seasonal viruses and protection against the novel variant.
© 2011 The Authors. Clinical Microbiology and Infection © 2011 European Society of Clinical Microbiology and Infectious Diseases.