Incidence of symptomatic A(H1N1)pdm09 influenza during the pandemic and post-pandemic periods in a rural Indian community

BACKGROUND: Data on influenza illness rates with population denominators are needed to quantify overall morbidity and to prioritize public health intervention strategies.

METHODS: The rates of influenza A(H1N1)pdm09 infection during pandemic phases were determined in a longitudinal community cohort study as part of an influenza vaccine study in a rural community of North India.

RESULTS: During the 711,731 person-weeks of surveillance, a total of 1410/7571 (19%) febrile acute respiratory illness cases were positive for influenza. Of these, 749 (53%) were influenza A(H1N1)pdm09, 643 (46%) influenza B, and 18 (1%) influenza A (H3N2). The overall incidence rate of influenza-associated febrile acute respiratory illness was 128/1000 person-years. The incidence rates of influenza A(H1N1)pdm09 were high during both the pandemic phase (179/1000 person-years; November 2009 to January 2010) and post-pandemic phase (156/1000 person-years; August to October 2010), with children<18 years of age being at the greatest risk of influenza infection in the community.

CONCLUSIONS: These findings provide important information for planning clinical and public health intervention strategies to mitigate the impact of influenza epidemics.