Repeatability and timing of tropical influenza epidemics

Much of the world experiences influenza in yearly recurring seasons, particularly in temperate areas. These patterns can be considered repeatable, occurring predictably and consistently. In tropical areas, including southeast Asia, this consistency is less conspicuous. This study aimed to assess repeatability of influenza in Vietnam. A mathematical model was developed incorporating periods of increased transmission, and fit to data from sentinel hospitals throughout Vietnam as well as four temperate locations. Repeatability was evaluated through the variance of the timings of peak transmission. Model fits from Vietnam show high variance (sd = 70-171) in peak transmission timing; peaks occurred at irregular intervals and throughout different times of year. Fits from temperate locations showed regular, annual epidemics in winter months, with low variance in peak timings (sd = 33-74). This suggests that influenza patterns are not repeatable or seasonal in Vietnam. Influenza prevention in Vietnam therefore cannot rely anticipating regularly occurring outbreaks.