• Henry C Baggett, Malinee Chittaganpitch, Somsak Thamthitiwat, Prabda Prapasiri, Sathapana Naorat, Pongpun Sawatwong, Darunee Ditsungnoen, Sonja J Olsen, James M Simmerman, Prasong Srisaengchai, Somrak Chantra, Leonard F Peruski, Pathom Sawanpanyalert, Susan A Maloney, and Pasakorn Akarasewi.
• International Emerging Infections Program, Thailand Ministry of Public Health (MOPH) - U.S. Centers for Disease Control and Prevention Collaboration, Nonthaburi, Thailand. hbaggett@cdc.gov
• Plos One. 2012 Jan 1; 7 (11): e48609.

BackgroundData on the burden of the 2009 influenza pandemic in Asia are limited. Influenza A(H1N1)pdm09 was first reported in Thailand in May 2009. We assessed incidence and epidemiology of influenza-associated hospitalizations during 2009-2010.MethodsWe conducted active, population-based surveillance for hospitalized cases of acute lower respiratory infection (ALRI) in all 20 hospitals in two rural provinces. ALRI patients were sampled 1∶2 for participation in an etiology study in which nasopharyngeal swabs were collected for influenza virus testing by PCR.ResultsOf 7,207 patients tested, 902 (12.5%) were influenza-positive, including 190 (7.8%) of 2,436 children aged <5 years; 86% were influenza A virus (46% A(H1N1)pdm09, 30% H3N2, 6.5% H1N1, 3.5% not subtyped) and 13% were influenza B virus. Cases of influenza A(H1N1)pdm09 first peaked in August 2009 when 17% of tested patients were positive. Subsequent peaks during 2009 and 2010 represented a mix of influenza A(H1N1)pdm09, H3N2, and influenza B viruses. The estimated annual incidence of hospitalized influenza cases was 136 per 100,000, highest in ages <5 years (477 per 100,000) and >75 years (407 per 100,000). The incidence of influenza A(H1N1)pdm09 was 62 per 100,000 (214 per 100,000 in children <5 years). Eleven influenza-infected patients required mechanical ventilation, and four patients died, all adults with influenza A(H1N1)pdm09 (1) or H3N2 (3).ConclusionsInfluenza-associated hospitalization rates in Thailand during 2009-10 were substantial and exceeded rates described in western countries. Influenza A(H1N1)pdm09 predominated, but H3N2 also caused notable morbidity. Expanded influenza vaccination coverage could have considerable public health impact, especially in young children.

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