Prevalence of Seropositivity to Pandemic Influenza A/H1N1 Virus in the United States following the 2009 Pandemic

<div><h3>Background</h3><p>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.</p> <h3>Methods</h3><p>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.</p> <h3>Results</h3><p>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.</p> <h3>Conclusions</h3><p>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.</p> </div

Vaccination and adjusted estimates of the increase in seropositivity from baseline to December 2009 due to natural infection with A(H1N1) pdm2009.

<p>SE = Standard error, CI = Confidence interval.</p>a<p>Age-standardized to the US population.</p>b<p>Adjusting for overlap from vaccination among people already infected with A(H1N1)pdm09. See methods for description of adjustment.</p>c<p>After adjusting for vaccination and assuming the following HI test characteristics: Sensitivity = 75%, Specificity = 97% (for ages <65) or 94% (for ages 65+).</p

Overall prevalence of seropositivity to A(H1N1)pdm09 at baseline and December 2009, by age group.

a<p>Estimates were weighted using the adjusted sample weight for this sample, WTH1N1.</p>b<p>Assuming no pre-existing seropositivity at baseline.</p>c<p>Age-standardized to the U.S. Census Bureau population estimates as of July 1, 2009 by age groups in the table.</p

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

<p>Comparison of the reverse cumulative distribution of HI titers between baseline and 2009 serum specimens, by age group.</p

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.

<p>^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.</p

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.

<p>Values are age–standardized to the US population.</p

Description and univariate analysis of risk factors for mortality among HIV non-infected and HIV-infected children <24 months of age hospitalized with LRTI.

<p>*<i>p<0.20;</i></p><p>**<i>p<0.05.</i></p>†<p> <i>Missing observations for >10%;</i></p

Distribution of RISC scores and screening performance in children <24 months of age with and without HIV infection, hospitalized with LRTI in Soweto, South Africa 1998–2001.

<p>*For mortality, using a cutoff at the corresponding score value.</p

Distribution of RISC score stratified by mortality for children <24 months hospitalized with LRTI, (A) without HIV infection and (B) with HIV infection.

<p>Distribution of RISC score stratified by mortality for children <24 months hospitalized with LRTI, (A) without HIV infection and (B) with HIV infection.</p

Independent risk factors for mortality in children <24 months hospitalized with LRTI, by HIV status.

<p>Independent risk factors for mortality in children <24 months hospitalized with LRTI, by HIV status.</p