Seroprevalence study in Vojvodina (Serbia) following 2009 pandemic influenza A(H1N1)V.

Introduction: The seroprevalence study was performed in Vojvodina during May and June 2010 in order to asses the effects of the 2009 pandemic influenza A(H1N1)v epidemic on herd immunity. It was a part of the Serbian Ministry of Health funded nationwide study.

Objective: Prevalence of antibodies against 2009 pandemic influenza A(H1N1)v was determined in a 1% sample of the population monitored for influenza-like illness and acute respiratory infections in Vojvodina through sentinel surveillance system.

Methods: The study sample involved a total of 1004 inhabitants of Vojvodina. The control group consisted of randomly selected and age-adjusted 1054 sera collected in the pre-pandemic period. Sera were tested by the reaction of hemagglutination inhibition using influenza A/California/7/2009 (H1N1) antigen in dilution from 1:8 to 1:256. Antibody titers > or = 1:32 and > or = 1:8 were considered protective and diagnostic, respectively.

Results: The differences between control and study sera in all age groups were significant for both diagnostic > or = 1/8 and protective titres > or = 1/32 of hemagglutination inhibition antibodies (chi square test, p < 0.001). The highest percentage of seropositive subjects was registered in the age group 15-19 years followed by children aged 5-14 years. Both diagnostic and protective titres were about twice higher in the vaccinated as compared to the non-vaccinated group. There were no statistically significant differences in seroprevalence between seven districts of Vojvodina.

Conclusion: The 2009 pandemic influenza A(H1N1)v epidemic significantly influenced the herd immunity in our population regardless of low immunization coverage with highest immunity levels in adolescents aged 15-19 years and with similar herd immunity levels in all the regions in the province six months after the outbreak.