Influenza Circulation and the Burden of Invasive Pneumococcal Pneumonia during a Non-pandemic

Background. Animal models and data from influenza pandemics suggest that influenza infection predisposes individuals to pneumococcal pneumonia. Influenza may contribute to high winter rates of pneumococcal pneu-monia during non-pandemic periods, but the magnitude of this effect is unknown. With use of United States surveillance data during 1995–2006, we estimated the association between influenza circulation and invasive pneumococcal pneumonia rates. Methods. Weekly invasive pneumococcal pneumonia incidence, defined by isolation of pneumococci from normally sterile sites in persons with clinical or radiographic pneumonia, was estimated from active population-based surveillance in 3 regions of the United States. We used influenza virus data collected by World Health Organization collaborating laboratories in the same 3 regions in seasonally adjusted negative binomial regression models to estimate the influenza-associated fraction of pneumococcal pneumonia. Results. During ∼185 million person-years of surveillance, we observed 21,239 episodes of invasive pneu-mococcal pneumonia; 485,691 specimens were tested for influenza. Influenza circulation was associated with 11%– 14 % of pneumococcal pneumonia during periods of influenza circulation and 5%–6 % overall. In 2 of 3 regions, the association was strongest when influenza circulation data were lagged by 1 week. Conclusions. During recent seasonal influenza epidemics in the United States, a modest but potentially pre