Estimating The Impact of Public Health Interventions on COVID Mortality in The United States Using Reductions in Influenza Mortality as an Indicator Of Non-Pharmaceutical Infection Control

Background The synergistic nature of non-pharmaceutical interventions (NPIs) for control of COVID makes it difficult to assess the efficacy of any individual strategy. This study uses influenza mortality reduction (IMR) during the pandemic as an indicator of the aggregate efficacy of NPIs in limiting the transmission of respiratory diseases and uses it to assess their impact on COVID mortality. Methods Age-adjusted COVID mortality for US states were modeled as a function of four variables: COVID mortality prior to the introduction of NPIs in March of 2020, vaccination rates through the end of 2023, IMR for 2020-21 and 2021-22 as compared to the 2015–2019 flu seasons, and population density. To estimate the impact of NPI’s, the resulting model was used with a Monte Carlo simulation to generate a distribution of COVID deaths under the counterfactual condition of an IMR of zero. Results A simple linear model with only these variables explained 69% of the state-to-state variability in age adjusted COVID mortality. The resulting model suggests that NPI’s alone prevented 840,000 COVID related deaths in the United States over the course of the pandemic. Conclusions There does not seem to be likely explanation for the observed reduction in influenza mortality other than NPIs introduced to control COVID. The strong explanatory power of the linear model tends to support that assertion. These results demonstrate the utility of IMRs as an indicator of the aggregate impact of NPIs for controlling transmission of respiratory infections, including COVID and provide strong evidence of their critical role in reducing the health impact of the pandemic in the United States.