Prevalence of 2009 Pandemic Influenza A (H1N1) Virus Antibodies, Tampa Bay Florida — November–December, 2009

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.

The dynamics of infection and the persistence of immunity to A(H1N1)pdm09 virus in Israel

“…Most of the results discussed earlier correlate well with previous publications describing the dynamics of the pandemic virus infections in different countries and age distribution 1, 11, 18, 19, 20, 21, 22…”

“…The cutoff value selected for a positive result was 1:40. HI antibody titers of ≥1:40 had been shown to correlate with reduction of 50% of the risk of contracting an influenza infection or disease9, 10, 11 and is frequently used as proxy for immunity. Thus, the term seroprevalence in this study refers to prevalence of antibody at titers of ≥1:40.…”

Influenza as a Proportion of Pneumonia Mortality: United States, 1959–2009

“…Throughout the early 2000s, there was also concern about the possibility of human transmission of H5N1 “bird flu” (Oxford 2005). The 2009 flu season corresponds to the H1N1 pandemic, which affected up to one-fifth of the US population (Shrestha et al 2011; Cox et al 2011). Thus, the use of influenza on death certificates seems to reflect its presence in the medical news.…”

Influenza Mortality in the United States, 2009 Pandemic: Burden, Timing and Age Distribution

“…The pandemic was declared over on 10 August 2010 [4]. Approximately 20% of the US population contracted influenza during the pandemic [5], [6]. In 2009, there were 53,692 pneumonia and influenza deaths, making this combination the eighth leading cause of death [7]; in 2010 there were 50,003 pneumonia and influenza deaths (ninth leading cause) [8].…”