Pandemic influenza H1N1 2009 in Thailand

ABSTRACT

Background: Developing a quantitative understanding of pandemic influenza dynamics in SouthEast Asia is important for informing future pandemic planning. Hence, transmission dynamics ofinfluenza A/H1N1 were determined across space and time in Thailand.Methods: Dates of symptom onset were obtained for all daily laboratory-confirmed cases of influenzaA/H1N1pdm in Thailand from 3 May 2009 to 26 December 2010 for four different geographicregions (Central, North, North-East, and South). These data were analysed using a probabilisticepidemic reconstruction, and estimates of the effective reproduction number, R(t), were derivedby region and over time.Results: Estimated R(t) values for the first wave peaked at 1.54 (95% CI 1.42-1.71) in the Centralregion and 1.64 (95% CI 1.38-1.92) in the North, whilst the corresponding values in the North-Eastand the South were 1.30 (95% CI 1.17-1.46) and 1.39 (95% CI 1.32-1.45) respectively. As theR(t) in the Central region fell below one, the value of R(t) in the rest of Thailand increased aboveone. R(t) was above one for 30 days continuously through the first wave in all regions of Thailand.During the second wave R(t) was only marginally above one in all regions except the South.Conclusions: In Thailand, the value of R(t) varied by region in the two pandemic waves. HigherR(t) estimates were found in Central and Northern regions in the first wave. Knowledge of regionalvariation in transmission potential is needed for predicting the course of future pandemics and foranalysing the potential impact of control measures: