Prevalence of seropositivity to pandemic influenza A/H1N1 virus in the United States following the 2009 pandemic

Abstract

Background: 2009 pandemic influenza A/H1N1 (A(H1N1)pdm09) was first detected in the United States in April 2009 and resulted in a global pandemic. We conducted a serologic survey to estimate the cumulative incidence of A(H1N1)pdm09 through the end of 2009 when pandemic activity had waned in the United States.

Methods: We conducted a pair of cross sectional serologic surveys before and after the spring/fall waves of the pandemic for evidence of seropositivity (titer ≥40) using the hemagglutination inhibition (HI) assay. We tested a baseline sample of 1,142 serum specimens from the 2007-2008 National Health and Nutrition Examination Survey (NHANES), and 2,759 serum specimens submitted for routine screening to clinical diagnostic laboratories from ten representative sites.

Results: The age-adjusted prevalence of seropositivity to A(H1N1)pdm09 by year-end 2009 was 36.9% (95%CI: 31.7-42.2%). After adjusting for baseline cross-reactive antibody, pandemic vaccination coverage and the sensitivity/specificity of the HI assay, we estimate that 20.2% (95%CI: 10.1-28.3%) of the population was infected with A(H1N1)pdm09 by December 2009, including 53.3% (95%CI: 39.0-67.1%) of children aged 5-17 years.

Conclusions: By December 2009, approximately one-fifth of the US population, or 61.9 million persons, may have been infected with A(H1N1)pdm09, including around half of school-aged children.

Figure 1. National trends in influenza–like illness (ILI) (solid line)^a and coverage with A(H1N1)pdm09 vaccine (dashed line)^b in the United States, and the period of sera collection.

^a Source: U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet). ^b Source: CDC estimates from combined Behavioral Risk Factors Surveillance System (BRFSS) and National 2009 H1N1 Flu Survey (NHFS) data.

Figure 2. Comparison of the reverse cumulative distribution of HI titers between baseline and 2009 serum specimens, by age group.

Figure 3. Geographic variation in prevalence of seropositivity to A(H1N1)pdm09 in 2009 (circles) and the proportion of the population reporting A(H1N1)pdm09 vaccination (squares) during the same time period, by state with 95% confidence intervals.

Values are age–standardized to the US population.

Figure 4. Estimated seroprevalence to A(H1N1)pdm09 through December 2009 by source and age group, adjusted for the sensitivity and specificity of HI assay.