Estimates of the prevalence of pandemic (H1N1) 2009, United States, April–July 2009

DOI: 10.3201/eid1512.091413
Suggested citation for this article: Reed C, Angulo FJ, Swerdlow DL, Lipsitch M, Meltzer MI, Jernigan D, et al. Estimates of the prevalence of pandemic (H1N1) 2009, United States, April–July 2009. Emerg Infect Dis [serial on the Internet]. 2009 Dec; [Epub ahead of print]

Estimates of the Prevalence of Pandemic (H1N1) 2009, United States, April–July 2009
Carrie Reed, Frederick J. Angulo, David L. Swerdlow, Marc Lipsitch, Martin I. Meltzer, Daniel Jernigan, and Lyn Finelli

Author affiliations: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (C. Reed, F.J. Angulo, D.L. Swerdlow, M.I. Meltzer, D. Jernigan, L. Finelli); and Harvard School of Public Health, Boston, Massachusetts, USA (M. Lipsitch)
Through July 2009, a total of 43,677 laboratory-confirmed cases of influenza A pandemic (H1N1) 2009 were reported in the United States, which is likely a substantial underestimate of the true number. Correcting for under-ascertainment using a multiplier model, we estimate that 1.8 million–5.7 million cases occurred, including 9,000–21,000 hospitalizations.

Human cases of influenza A pandemic (H1N1) 2009 were first identified in the United States in April 2009 (1,2). By the end of July, >40,000 laboratory-confirmed infections had been reported, representing only a fraction of total cases. Persons with influenza may not be included in reported counts for a variety of reasons, including the following: not all ill persons seek medical care and have a specimen collected, not all specimens are sent to a public health laboratory for confirmatory testing with reverse transcription–PCR (rapid point-of-care testing can not differentiate pandemic (H1N1) 2009 from other strains), and not all specimens will give positive results because of the timing of collection or the quality of the specimen. To better estimate the prevalence of pandemic (H1N1) 2009 during April–July 2009 in the United States, we created a simple multiplier model that adjusts for these sources of under-ascertainment.

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