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

Background: Little is known about global variation in early epidemic growth rates and effective reproduction numbers (R_e) of seasonal influenza. We aimed to estimate global variation in R_e of influenza type A and B during a single period.

Methods: Country 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 R_e estimates for a five-week moving window, advanced by week. Associations of R_e 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.

Results: Time series were included for 119 of 169 available countries. There were 100 countries with influenza A and 79 with B epidemics. Median R_e for both influenza A and B epidemics was 1.23 (ranges: A 1.10, 1.60; B 1.06, 1.58). R_e 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.

Conclusion: The R_e of influenza A and B were similar. Only the R_e of influenza B was associated with country characteristics, which was increasing with distance from the equator. The approach may be suitable for continuous R_e surveillance.