Influenza Mortality as an Indicator of the Efficacy of COVID-Related, Non-pharmaceutical Interventions to Reduce the Spread of Respiratory Infections (preprint)

ABSTRACT

Background Non-pharmaceutical interventions (NPIs) have been criticized as ineffective in preventing COVID. Because it is a new disease with NPIs introduced almost immediately, we have no way to easily and directly evaluate the counterfactual of non-intervention. We do, however, have excellent historic data on influenza, a respiratory disease with similar patterns of transmission and a well-established CDC surveillance system in the US. These data provide us an excellent way to indirectly evaluate the efficacy of these interventions and factors influencing that efficacy. Results During the three seasons prior to COVID from 2016-17 to 2018-19, the mean total US influenza mortality was 9,917 deaths per season. During the pandemic, influenza mortality was reduced by 91 percent in the 2020-21 and 69 percent in the 2021-22 influenza seasons relative to historical levels. The corresponding decreases for the pediatric population were 99 percent and 67 percent. At the state level, the average drop in mortality over the two flu seasons of the pandemic was strongly correlated with the percent of the 2020 presidential votes cast for Joe Biden in each state. Conclusions These data provide strong evidence that COVID NPIs dramatically reduced the spread of influenza. Given its similar routes of transmission, these results support the assertion that these interventions also substantially reduced COVID transmission, morbidity, and mortality. The effectiveness of NPIs increased in proportion to the size of the Democratic electorate in each state. This could reflect more effective NPI strategies in states with Democratic leanings or better compliance with NPIs in those states.