Understanding the epidemiology of human influenza in Viet Nam is important for developing local policies and also for understanding the dynamics of influenza in tropical and subtropical southeast Asia. I have analysed an 18 year time-series of influenza-like-illness (ILI) surveillance data, and assessed the relationship of this time-series with climate variables and with sentinel influenza virus surveillance data. I also conducted a study of influenza A/H1N1 transmission within households.
ILI notifications in Viet Nam show a latitudinal gradient, with seasonality in the north but no seasonal pattern observed in low lying areas of central and southern Viet Nam. Seasonality is however observed in the elevated provinces of central Viet Nam, suggesting that the seasonal patterns are driven by climate. Principal component analysis finds that temperature and absolute humidity (AH) are positively correlated and together explain around 59% of total climatic variance, and that there is a strong latitudinal gradient in these variables. Regression tree analysis shows that provinces with strong seasonality of AH have strong ILI seasonality. Although virological surveillance data are limited, increases in ILI notifications are associated with an increase in the proportion of upper respiratory tract swabs that are influenza positive. In a prospective study of H1N1/2009 transmission in a household-based cohort, 11 of 59 household contacts were infected, giving a household secondary infection risk of 18.6% (95%CI 10.7-30.4%), but 5 (45%) did not develop symptoms. Virus genetic sequencing indicated that 10 of the 11 secondary cases (91%) were probably infected within the household rather than from the community. This research provides new insights into the seasonality and climatic determinants of ILI and influenza epidemiology in Viet Nam, and on the transmission of influenza within households. The findings are valuable for national influenza control policies and also add to the current state of knowledge of influenza epidemiology
