Estimates of the Number of Human Infections With Influenza A(H3N2) Variant Virus, United States, August 2011-April 2012

Methods. To account for this underascertainment, we adapted a multiplier model created at the beginning of the influenza A(H1N1) 2009 pandemic to estimate the true burden of H3N2v illness. Data to inform each of these parameters came from the literature and from special projects conducted during the 2009 H1N1 pandemic and the 2010-2011 influenza season. The multipliers were calculated as the simple inverses of the proportions at each step, and we accounted for variability and uncertainty in model parameters by using a probabilistic or Monte Carlo approach. Results. We estimate that the median multiplier for children was 200 (90% range, 115-369) and for adults was 255 (90% range, 152-479) and that 2055 (90% range, 1187-3800) illnesses from H3N2v virus infections may have occurred from August 2011 to April 2012, suggesting that the new virus was more widespread than previously thought. Conclusions. Illness from this variant influenza virus was more frequent than previously thought. Continued surveillance is needed to ensure timely detection and response to H3N2v virus infections.