Does sunlight drive seasonality of TB in Vietnam? A retrospective environmental ecological study of Tuberculosis seasonality in Vietnam from 2010-2015
Abstract Background Tuberculosis (TB) is a major global health burden, with an estimated quarter of the world’s population being infected. The World Health Organization (WHO) launched the “End TB Strategy” in 2014 with emphasis placed on knowing the epidemic. Vietnam is 12th in the world ranking of high burden countries by WHO definition. TB spatial and temporal patterns have been observed globally with evidence of Vitamin D playing a role in the seasonality. We explored the presence of temporal and spatial clustering of TB in Vietnam and their determinants to aid public health measures.Methods Data were collected by the National TB program of Vietnam from 2010-2015 and linked to the following datasets: socio-demographic characteristics; climatic variables; influenza-like-illness (ILI) incidence; geospatial data. The TB dataset was aggregated by province and quarter. Descriptive time series analyses using LOESS regression were completed per province to determine seasonality and trend. Harmonic regression was used to determine the amplitude of seasonality by province. A mixed-effect linear model was used with province and year as random effects and all other variables as fixed effects.Results There were 610,676 cases of TB notified between 2010-2015 in Vietnam. Heat maps of TB incidence per quarter per province showed substantial temporal and geospatial variation. Time series analysis demonstrated seasonality throughout the country, with peaks in spring/summer and troughs in autumn/winter. Incidence was consistently higher in the south. However, relative seasonal amplitude was more pronounced in the north. Mixed-effect linear model confirmed that TB incidence was associated with time and latitude. Of the demographic, socio-economic and health related variables, population density, percentage of those under 15 years of age, and HIV infection prevalence per province were associated with TB incidence. Of the climate variables, absolute humidity, average temperature and sunlight were associated with TB incidence.Conclusion Incidence decreased and the relative seasonal amplitude of TB increased with latitude in Vietnam. Temporal and spatial hotspots were found. Seasonality of TB was associated with reduced hours of sunlight at a lag of six months.