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Abstract

Background: Varicella, or chickenpox, is a ubiquitous, directly-transmitted childhood infection that occurs in individuals infected by varicella-zoster virus (VZV). Despite the existence of an effective vaccine, annual outbreaks of varicella are common throughout many parts of the world; the illness thus remains a global health concern. Thailand is among the list of countries that continue to struggle with varicella due to accessibility issues. The transmission of varicella and other infectious diseases relies on a variety of interacting factors, including the gathering of children in school and the conditions of the climate. While the relationship between other common infectious diseases and the climate is well-established, varicella has received little attention concerning this topic. Anthropogenic climate change is projected to alter current climate trends, so it is imperative that these relationships are understood and properly acted upon before irreversible changes occur. Methods: In order to better understand the relationship between the climate and varicella transmission, this study sought to: (1) characterize patterns in varicella incidence in Thailand and (2) estimate the effect of two climate variables, temperature and precipitation, on varicella transmission. A time-series Susceptible-Infected-Recovered (TSIR) model was fit to incidence and demographic data from Thailand to produce estimated transmission rates within Thai provinces. Using these transmission rates and climate data from over 70 Thai provinces between 2003 and 2015, a linear regression model was run to estimate the effects of temperature and precipitation on transmission. Results: Varicella incidence is highly regular in Thailand, with annual peaks occurring in March—which marks the end of the school year for children. The varicella incidence trends matched patterns of neither temperature nor precipitation. The highest monthly temperatures in Thailand occur just after incidence peaks, while the lowest monthly precipitation levels align with the periods of highest incidence. According to the linear regression model, both temperature and precipitation have significant, negative relationships with varicella transmission. The estimated coefficient on temperature (-0.007, s.e. 0.002) is notably larger than the estimated coefficient on precipitation (-7.86e-04, s.e. 2.87e-04). Conclusions: The results indicate that there is likely a complex interaction of factors that collectively influence varicella transmission in Thailand. Alone, realistic changes in neither temperature nor precipitation are likely to alter varicella incidence to an observable degree in this geographic region. Nevertheless, public health efforts must remain focused on combatting the spread of varicella in areas where vaccination coverage remains low. It is difficult to predict how the countless, variable effects of climate change around the world will interact to effect the incidence of varicella and other directly-transmitted infectious diseases. Future studies should continue exploring the relationships between these infections and the climate as they may prove to be increasingly relevant in the near future.