Abstract
BackgroundPandemic influenza A(H1N1)pdm09 emerged in Thailand in 2009. A prospective longitudinal adult cohort and household transmission study of influenza-like illness (ILI) was ongoing in rural Thailand at the time of emergence. Symptomatic and subclinical A(H1N1)pdm09 infection rates in the cohort and among household members were evaluated.MethodsA cohort of 800 Thai adults underwent active community-based surveillance for ILI from 2008–2010. Acute respiratory samples from ILI episodes were tested for A(H1N1)pdm09 by qRT-PCR; acute and 60-day convalescent blood samples were tested by A(H1N1)pdm09 hemagglutination inhibition assay (HI). Enrollment, 12-month and 24-month follow-up blood samples were tested for A(H1N1)pdm09 seroconversion by HI. Household members of influenza A-infected cohort subjects with ILI were enrolled in household transmission investigations in which day 0 and 60 blood samples and acute respiratory samples were tested by either qRT-PCR or HI for A(H1N1)pdm09. Seroconversion between annual blood samples without A(H1N1)pdm09-positive ILI was considered as subclinical infection.ResultsThe 2-yr cumulative incidence of A(H1N1)pdm09 infection in the cohort in 2009/2010 was 10.8% (84/781) with an annual incidence of 1.2% in 2009 and 9.7% in 2010; 83.3% of infections were subclinical (50% in 2009 and 85.9% in 2010). The 2-yr cumulative incidence was lowest (5%) in adults born ≤1957. The A(H1N1)pdm09 secondary attack rate among household contacts was 47.2% (17/36); 47.1% of these infections were subclinical. The highest A(H1N1)pdm09 secondary attack rate among household contacts (70.6%, 12/17) occurred among children born between 1990 and 2003.ConclusionSubclinical A(H1N1)pdm09 infections in Thai adults occurred frequently and accounted for a greater proportion of all A(H1N1)pdm09 infections than previously estimated. The role of subclinical infections in A(H1N1)pdm09 transmission has important implications in formulating strategies to predict and prevent the spread of A(H1N1)pdm09 and other influenza virus strains.
Highlights
Pandemic influenza A(H1N1)pdm09 emerged in North America in March 2009 and rapidly spread to other parts of the world, eventually replacing previous seasonal H1N1 virus strains
Several household studies reported a wide range (8% to 45%) of secondary attack rates [7,8,9,10]. These studies were mainly conducted during the early stages of the pandemic (April to August 2009) based on household investigations usually triggered by sentinel A(H1N1)pdm09 cases
Ethics Statement The study was approved by the institutional review boards of the Thai Ministry of Public Health (MOPH), Walter Reed Army Institute of Research (WRAIR), Naval Medical Research Unit No 2 (NAMRU-2), US Army Medical Research and Materiel Command, University of Iowa, and University of Florida
Summary
Pandemic influenza A(H1N1)pdm emerged in North America in March 2009 and rapidly spread to other parts of the world, eventually replacing previous seasonal H1N1 virus strains. Several studies from different countries established a pattern of higher A(H1N1)pdm infection rates in school-age children relative to young adults, with older adults having the lowest infection rates [1,2,3,4,5,6]. These studies were primarily community-based serological surveys that relied on non-prospective cohort designs and convenience sampling rather than sampling of prospective longitudinal cohorts. Symptomatic and subclinical A(H1N1)pdm infection rates in the cohort and among household members were evaluated
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