Abstract
BackgroundIn 2009, a novel influenza virus (2009 pandemic influenza A (H1N1) virus (pH1N1)) caused significant disease in the United States. Most states, including Florida, experienced a large fall wave of disease from September through November, after which disease activity decreased substantially. We determined the prevalence of antibodies due to the pH1N1 virus in Florida after influenza activity had peaked and estimated the proportion of the population infected with pH1N1 virus during the pandemic.MethodsDuring November-December 2009, we collected leftover serum from a blood bank, a pediatric children's hospital and a pediatric outpatient clinic in Tampa Bay Florida. Serum was tested for pH1N1 virus antibodies using the hemagglutination-inhibition (HI) assay. HI titers ≥40 were considered seropositive. We adjusted seroprevalence results to account for previously established HI assay specificity and sensitivity and employed a simple statistical model to estimate the proportion of seropositivity due to pH1N1 virus infection and vaccination.ResultsDuring the study time period, the overall seroprevalence in Tampa Bay, Florida was 25%, increasing to 30% after adjusting for HI assay sensitivity and specificity. We estimated that 5.9% of the population had vaccine-induced seropositivity while 25% had seropositivity secondary to pH1N1 virus infection. The highest cumulative incidence of pH1N1 virus infection was among children aged 5–17 years (53%) and young adults aged 18–24 years (47%), while adults aged ≥50 years had the lowest cumulative incidence (11–13%) of pH1N1 virus infection.ConclusionsAfter the peak of the fall wave of the pandemic, an estimated one quarter of the Tampa Bay population had been infected with the pH1N1 virus. Consistent with epidemiologic trends observed during the pandemic, the highest burdens of disease were among school-aged children and young adults.
Highlights
The 2009 pandemic influenza A (H1N1) virus was first identified in April 2009 and caused widespread illness in the United States and around the world [1]
Tampa Bay experienced a gradual increase in influenza activity in the spring and summer of 2009, followed by a large fall wave of influenza activity that peaked in late October and decreased steadily thereafter (Figure 1)
Because serology cannot differentiate between antibodies produced by virus infection and response to vaccination, we developed a simple statistical model to estimate the proportion of seropositive results due to pandemic influenza A (H1N1) virus (pH1N1) vaccination coverage (Appendix S2)
Summary
The 2009 pandemic influenza A (H1N1) virus (pH1N1) was first identified in April 2009 and caused widespread illness in the United States and around the world [1]. The Centers for Disease Control and Prevention (CDC) estimated that during the pandemic, 14–29% of the US population had a clinical case of influenza [2]. This estimate excluded subclinical cases which may have accounted for 24–36% of all infections [3,4,5,6]. Estimating the total number of pH1N1 virus infections in Tampa Bay that were acquired during this time period presented several challenges. We determined the prevalence of antibodies due to the pH1N1 virus in Florida after influenza activity had peaked and estimated the proportion of the population infected with pH1N1 virus during the pandemic
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