As an international traveling hub of South-East Asia, Thailand was one of the countries hardest and earliest hit by the influenza A (H1N1) 2009 pandemic. In order to understand the epidemic spread in the country, we conducted community-based surveys in metropolitan, urban, and rural areas using questionnaire interviews. We also determined sero-positive rates from randomly selected samples within the surveyed population. Recalled incidences of fever and acute respiratory symptoms in the survey correlated well with systematic reports of 2009 pandemic influenza cases from hospitals in the same areas, giving a ratio of total cases extrapolated from the surveyed data for persons who sought medical attention reported in the hospital-based surveillance system at 275:1. Conducting a large scale survey of the influenza outbreak is time consuming and also can be difficult to complete in a short time. Therefore, we used the survey for monitoring the outbreak of respiratory disease in the early pandemic phase. The seroprevalence rate was 8 to 10%, with higher rate for younger age groups, and suggests that sufficient herd immunity may have been reached in Thailand, especially in urban areas, while others may still be vulnerable to the second wave of the pandemic.
Key words: Pandemic, influenza, survey, Thailand.
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