Assessing the interactions between COVID-19 and influenza in the United States

Casey M Zipfel; Shweta Bansal

is the (which was not peer-reviewed) The copyright holder for this preprint . tion, likely because there were no major shifts in humidity levels during the influenza season. We also tested whether a shift in the antigenic makeup of circulating influenza strains was explanatory. Early in the influenza season, influenza B was dominant, but since early February, influenza A H1N1 has been more prevalent. Influenza A H1N1 is expected to produce milder infections so this may explain the ILI reduction later in the season [32] . However, we found that influenza A dynamics were not significantly associated with ILI dynamics. We also considered whether there were changes in healthcare seeking that could have changed the apparent ILI dynamics, but we evaluated the number of total patients in ILINet, and found no increase in total patients within this timeframe (Appendix, Figure S2 ). Lastly, an increase in late vaccination may predict a smaller flu epidemic. However, CDC data from 2019-2020 shows that approximately 98% of the vaccine supply was distributed by January 1st 2020, and data from past seasons shows that over 90% of the cumulative flu vaccination coverage is reached by January 1st, 2020 [33, 34] . Thus we did not consider this hypothesis in our analysis.