The comparative burden and severity of seasonal and pandemic influenza, the results of the Flu Watch study England 2006-2011

Background: Evaluation of population impact of influenza including risk of infection, illness if infected, illness severity and consultation rates is fundamental to inform future control and prevention. We report the world’s first household-level national community cohort comparing seasonal and pandemic influenza. Methods: Using pre-and post-season serology, weekly illness reporting and RT-PCR identification of influenza from nasal swabs, we tracked the course of seasonal and pandemic influenza over five sequential years (England 2006-2011 - 5448 person-seasons’ follow up). We compare burden and severity of seasonal and pandemic strains. Results: Based on four-fold rises in strain-specific serology, on average influenza infected 18% (95% CI 16-22%) of unvaccinated people each winter. Amongst those infected there were 69 respiratory illnesses per 100 person influenza seasons compared to 44 per 100 in those not infected with influenza. The age-adjusted attributable rate of illness if infected was 23 illnesses per 100 person seasons (95% CI 13-34), suggesting the majority of influenza infections are asymptomatic. 25% (95% CI 18-35%) of all serologic infections had PCR-confirmed disease. 17% (95%CI 10-26% of PCR-confirmed cases had medically attended illness. These figures did not differ significantly comparing pandemic and seasonal influenza. Amongst PCR confirmed cases those infected with the pandemic strain had markedly less severe symptoms than those infected with seasonal H3N2. Conclusion: Seasonal influenza and the 2009 pandemic strain were characterised by similar high rates of mainly asymptomatic infection with most symptomatic cases self-managing without medical consultation. In the community the pandemic strain caused milder symptoms than seasonal H3N