When can we stop wearing masks? Agent-based modeling to identify when vaccine coverage makes nonpharmaceutical interventions for reducing SARS-CoV-2 infections redundant in indoor gatherings

Abstract

As vaccination efforts to combat the COVID-19 pandemic are ramping up worldwide, there are rising concerns that individuals will begin to eschew nonpharmaceutical interventions for preventing SARS-CoV-2 transmission and attempt to return to pre-pandemic normalcy before vaccine coverage levels effectively mitigate transmission risk. In the U.S.A., some governing bodies have already weakened or repealed guidelines for nonpharmaceutical intervention use, despite a recent spike in national COVID-19 cases and majority population of unvaccinated individuals. Recent modeling suggests that repealing nonpharmaceutical intervention guidelines too early into vaccine rollouts will lead to localized increases in COVID-19 cases, but the magnitude of nonpharmaceutical intervention effects on individual-level SARS-CoV-2 infection risk in fully- and partially-vaccinated populations is unclear. We use a previously-published agent-based model to simulate SARS-CoV-2 transmission in indoor gatherings of varying durations, population densities, and vaccination coverage levels. By simulating nonpharmaceutical interventions in some gatherings but not others, we were able to quantify the difference in SARS-CoV-2 infection risk when nonpharmaceutical interventions were used, relative to scenarios with no nonpharmaceutical interventions. We found that nonpharmaceutical interventions will often reduce secondary attack rates, especially during brief interactions, and therefore there is no definitive vaccination coverage level that makes nonpharmaceutical interventions completely redundant. However, the reduction effect on absolute SARS-CoV-2 infection risk conferred by nonpharmaceutical interventions is likely proportional to COVID-19 prevalence. Therefore, if COVID-19 prevalence decreases in the future, nonpharmaceutical interventions will likely still confer protective effects but potential benefits may be small enough to remain within "effectively negligible" risk thresholds.

Keywords: COVID-19; SARS-CoV-2; agent-based model; indoor transmission; nonpharmaceutical interventions; vaccine.

Figure 1.

At low population densities and gathering duration limits, nonpharmaceutical interventions to prevent infection and elevated vaccination rates consistently decrease the probability of observing ≥ 1 successful SARS-CoV-2 transmission events in simulations.

Figure 2.

Mean secondary attack rates in simulations indicate substantial variability in risk.

Figure 3.

Predicted secondary attack rates suggest that the combination of cloth face masks and 2-m social distancing during indoor gatherings of varying durations consistently reduces secondary attack rates by 55–58%. This effect was only modeled for vaccine efficacies of 60% and 80%.

Figure 4.

Estimated absolute risk of being infected with SARS-CoV-2 during 60-minute gatherings of varied sizes. Estimates were obtained by plugging Figure 3 predictions into Equation 3 with fixed COVID-19 prevalence and n values. a) Absolute risk of SARS-CoV-2 transmission given that 10 people attend the gathering. b) Absolute risk of SARS-CoV-2 transmission given that 20 people attend the gathering.