Global variation in early epidemic growth rates and reproduction number of seasonal influenza

Abstract

BackgroundLittle is known about global variation in early epidemic growth rates and effective reproduction numbers (Re) of seasonal influenza. We aimed to estimate global variation in Re of influenza type A and B during a single period. MethodsCountry influenza detection time series from September 2017 through January 2019 were obtained from an international database. Type A and B epidemics by country were selected on the basis of Re estimates for a five-week moving window, advanced by week. Associations of Re with absolute latitude, human development index, percent of the population aged <15 years, and percent of those living in rural areas in each country were assessed. ResultsTime series were included for 119 of 169 available countries. There were 100 countries with influenza A and 79 with B epidemics. Median Re for both influenza A and B epidemics was 1.23 (ranges: A 1.10, 1.60; B 1.06, 1.58). Re of influenza B but not A was independently associated with absolute latitude, increasing by 0.022 (95% confidence interval 0.002, 0.043) per 10 degrees. ConclusionThe Re of influenza A and B were similar. Only the Re of influenza B was associated with country characteristics, which was increasing with distance from the equator. The approach may be suitable for continuous Re surveillance.