Comparative community burden and severity of seasonal and pandemic influenza: results of the Flu Watch cohort study – The Lancet (2014)

Most Flu Is Asymptomatic

On average, roughly 20% of the unvaccinated had serologic evidence of influenza infection, but up to three quarters of the infected were asymptomatic. The proportions did not vary significantly between seasonal and pandemic influenzas. The pandemic H1N1 strain was associated with less severe symptoms than the seasonal H3N2 strain.

NEJM Journal Watch, 17 March 2014