Seroprevalence study of antibodies against influenza A(H1N1) 2009 virus after the second pandemic wave in Slovenia

Summary OBJECTIVE: The pandemic influenza (H1N1) 2009 virus, which combined genes from swine, human and avian influenza viruses, emerged in April 2009 and spread globally within the next month. The post-pandemic serological response to the influenza A(H1N1) 2009 virus in non-vaccinated individuals from the central part of Slovenia was studied. METHODS: After the second pandemic wave, 226 serum samples from healthy preschool children, students, health care workers and blood donors not previously vaccinated with pandemic vaccine were investigated by use of microneutralization assays (MN). Data on previous vaccinations with seasonal influenza vaccine and recent acute respiratory infections were collected. RESULTS: The overall rate of seropositivity to the pandemic influenza virus was high: 76.9% of the participants had antibody titres of ≥20. A titre of ≥40 was detected in 54% of the serum samples tested, the highest prevalence being in the 15–17 age group. There was no statistically significant difference in MN titres between individuals who had never been vaccinated with influenza vaccine and those vaccinated at least once. History of ARI was not more frequent in subjects with MN titres ≥20, ≥40 or ≥80 ( p -value 0.654, p -value 0.755 and p -value 1, respectively). CONCLUSIONS: High seroprevalence to pandemic influenza has been found in all age groups regardless of the absence of clinical picture compatible with acute respiratory infection. Previous vaccinations with seasonal influenza vaccines had no impact on serological response to the influenza A(H1N1) 2009 virus.