The authors investigated the association of simulated COVID-19 vaccine efficacy and the use of nonpharmaceutical interventions (NPIs) (reduced mobility, school closings, and use of face masks) with rates of infection, hospitalization, and death.
Study design
An established agent-based decision analytical model was used to simulate COVID-19 transmission and progression from March 24, 2020, to September 23, 2021. The model simulated COVID-19 spread in North Carolina, which has a population of 10.5 million people. A network of 1,017,720 agents was constructed from US Census data to represent the statewide population. Scenarios of vaccine efficacy (50% and 90%), vaccine coverage (25%, 50%, and 75% at the end of a 6-month distribution period), and NPIs maintained and removed during vaccine distribution were evaluated.
Outcomes
All vaccination scenarios with NPIs had lower risks than without NPI counterparts; these differences tended to increase with lower vaccine efficacy and coverage. In the worst-case vaccination scenario (50% efficacy, 25% coverage), a mean of 2,231,134 new infections occurred after vaccination began with NPIs removed, and a mean of 799,949 new infections occurred with NPIs maintained during 11 months. In contrast, in the best-case scenario (90% efficacy, 75% coverage), a mean of 527,409 new infections occurred with NPIs removed and a mean of 450,575 new infections occurred with NPIs maintained. With NPIs removed, lower efficacy (50%) and higher coverage (75%) reduced infection risk by a greater magnitude than higher efficacy (90%) and lower coverage (25%) compared with the worst-case scenario (mean absolute risk reduction of 13% and 8%, respectively).
Limitations
The findings are conditional on the authors' modeling assumptions and specification of vaccination and NPI scenarios. Other factors contributing to the study's limitations include the restricted geographic location and unique characteristics of the locale and the broad age category (20–64 years) that limited age specificity of outcomes among adults. Although variability in outcomes across replications was calculated, the results do not reflect uncertainty in the model structure and parameter specification. However, sensitivity analysis suggests the main findings are robust to variability in important infection transmission and mitigation parameters.
Clinical significance
Simulation outcomes suggest that removing NPIs, including mask-wearing and social distancing, while vaccines are distributed resulted in substantial increases in SARS-CoV-2 infections and COVID-19–associated hospitalizations and deaths. Under scenarios with low vaccine efficacy and coverage, premature removal of NPIs could result in a resurgence of infections with a magnitude exceeding that before vaccine distribution. As NPIs are removed, higher vaccination coverage with less efficacious vaccines can contribute to a larger reduction in risk of SARS-CoV-2 infection compared with more efficacious vaccines at lower coverage. These findings highlight the need for well-resourced and coordinated efforts to achieve vaccine coverage and continued adherence to NPIs before safely resuming many pre-pandemic activities.