Session:

Epidemiology & Public Health

Abstract No.:

53.024

Title:

Pandemic influenza A (H1N1) virus infections among villagers living in rural Thailand

Author(s):

B. Khuntirat¹, I.-K. Yoon¹, W. Krueger², M. Chittaganrnpitch³, K. Supawat³, P. Blair⁴, S. D. Putnam⁵, R. V. Gibbons¹, P. Sawanpanyalert³, G. L. Heil², J. A. Friary², G. C. Gray²; ¹Armed Forces Research Institute of Medical Sciences, Virology Dept, Bangkok/TH, ²College of Public Health and Health Professions, University of Florida, Environmental and Global Health Dept, Gainsville, FL/US, ³Thai National Institute of Health, Medical Sciences Dept, Nonthaburi/TH, ⁴Naval Health Research Center, Operational Infectious Diseases Dept, San Diego, CA/US, ⁵JMI Laboratories, Microbiology, North Liberty, IA/US

Abstract:

Background: The first confirmed case of pandemic influenza A (H1N1) virus (pH1N1) in Thailand was declared on 12 May 2009.  A prospective cohort study among approximately 800 adult subjects undergoing active community-based surveillance for influenza-like illness (ILI) in Kamphaeng Phet, Thailand was ongoing at the time of pandemic onset. Using this existing cohort, we aimed to determine the incidence of symptomatic and subclinical pH1N1 infection in adults.
Methods: Cohort subjects underwent active community-based surveillance by weekly visits to their homes. Subjects with influenza-like illness were tested by real-time reverse transcription-polymerase chain reaction (RT-PCR) on respiratory samples.  Hemagglutination inhibition (HI) assay for pH1N1 was carried out annually.  In addition, household members of cohort subjects with confirmed influenza A were evaluated.
Results: The pH1N1 virus was first detected in ill cohort subject in August 2009.  From 2009 to 2010, 71 cohort subjects developed HI seroconversion sera against pH1N1.  However, only 10 cohort subjects (3 in 2009 and 7 in 2010) were found to have symptomatic pH1N1 infection by real-time RT-PCR (9 out of 10 had HI seroconversion in acute/60-day convalescent sera).  Thirty-six household members (16 in 2009 and 20 in 2010; age range = 1-70 years) living with the pH1N1-positive cohort subject were evaluated.  Six of these household members (age range = 3-47 years) developed ILI within 4 days of the corresponding cohort case and were confirmed to have pH1N1 infection by PCR and HI seroconversion in acute/60-day convalescent sera.  Twelve household members without any ILI symptoms (age range = 1-70 years) developed pH1N1 seroconversion in acute/60-day convalescent sera; 7 members without symptoms (age range = 6-27 years) had a positive HI titer against pH1N1 from both acute and convalescent sera.
Conclusion: Subclinical or mild pH1N1 infections make up a majority (about 92%) of all pH1N1 infections in adults.  Household contacts had a high infection rate (70%) but a majority (76%) of these was subclinical or mild.  These findings demonstrate that subclinical or mild pH1N1 infection occurs frequently among human adults.