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. (c) 2022 The Author(s). Published by Elsevier Ltd on behalf of International Society for Infectious Diseases. This is an open access article under the CC BY-NC-ND license ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )

Automated summary

This study aimed to estimate the global variation in the effective reproduction numbers (R e ) of influenza type A and B. The results showed that the R e values of influenza A and B were similar, but only the R e value of influenza B was associated with country characteristics, with higher values observed at greater distances from the equator.