title: High rate of A(H1N1)pdm09 infections among rural Thai villagers, 2009-2010.
abstract: BACKGROUND
  Pandemic 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.
  
  
  METHODS
  A 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  Subclinical 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.
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Benjawan
authors.initials: B
authors.lastname: Khuntirat
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: In-Kyu
authors.initials: IK
authors.lastname: Yoon
authors.affiliation: National Institute of Health, Ministry of Public Health, Nonthaburi, Thailand.
authors.forename: Malinee
authors.initials: M
authors.lastname: Chittaganpitch
authors.affiliation: College of Public Health and Health Professions and Emerging Pathogens Institute, University of Florida, Gainesville, Florida, United States of America.
authors.forename: Whitney S
authors.initials: WS
authors.lastname: Krueger
authors.affiliation: National Institute of Health, Ministry of Public Health, Nonthaburi, Thailand.
authors.forename: Krongkaew
authors.initials: K
authors.lastname: Supawat
authors.affiliation: Naval Medical Research Center - Asia, Singapore, Singapore.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Blair
authors.affiliation: Naval Health Research Center, San Diego, California, United States of America.
authors.forename: Shannon D
authors.initials: SD
authors.lastname: Putnam
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Robert V
authors.initials: RV
authors.lastname: Gibbons
authors.affiliation: Department of Virology, Armed Forces Research Institute of Medical Sciences, Bangkok, Thailand.
authors.forename: Darunee
authors.initials: D
authors.lastname: Buddhari
authors.affiliation: National Institute of Health, Ministry of Public Health, Nonthaburi, Thailand.
authors.forename: Pathom
authors.initials: P
authors.lastname: Sawanpanyalert
authors.affiliation: College of Public Health and Health Professions and Emerging Pathogens Institute, University of Florida, Gainesville, Florida, United States of America.
authors.forename: Gary L
authors.initials: GL
authors.lastname: Heil
authors.affiliation: College of Public Health and Health Professions and Emerging Pathogens Institute, University of Florida, Gainesville, Florida, United States of America.
authors.forename: John A
authors.initials: JA
authors.lastname: Friary
authors.affiliation: College of Public Health and Health Professions and Emerging Pathogens Institute, University of Florida, Gainesville, Florida, United States of America.
authors.forename: Gregory C
authors.initials: GC
authors.lastname: Gray
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D058345:Asymptomatic Infections; D002648:Child; D002675:Child, Preschool; D005191:Family Characteristics; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000472:pathogenicity* / Q000502:physiology; D007251:Influenza, Human / Q000453:epidemiology* / Q000503:physiopathology / Q000635:transmission; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D011446:Prospective Studies; D012424:Rural Population; D013785:Thailand / Q000453:epidemiology