Estimates of the number of human infections with influenza A (H3N2) variant virus, United States, August 2011–April 2012

Background Thirteen human infections with an influenza A (H3N2)variant (H3N2v) virus containing a combination of gene segments not previously associated with human illness were identified in the United States from August 2011 to April 2012. Because laboratory confirmation of influenza virus infection is only performed for a minority of those ill and routine clinical tests may not identify H3N2v virus, the count of laboratory-confirmed H3N2v influenza virus infections underestimates the true burden of illness. Methods To account for this under-ascertainment, we adapted a multiplier model created at the beginning of the influenza A(H1N1)pdm09 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 2010–11 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 Using this approach, 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 2,055 (90% range 1,187–3,800) H3N2v 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.