This simulation was performed before the Delta mutant came out, but it probably still holds true if outbreak is defined as serious disease. All scenarios involve loosening social distancing restrictions. Note that if everyone wore masks at the highest levels for that area, only 25-50% of the people would have to be vaccinated with a moderately (80%) effective vaccine to avert another outbreak. Even if mask use drops 50% from prior levels, a moderately effective vaccine requires only 32-57% coverage to avert a new outbreak. However, if no one wears masks, vaccine coverage would have to be up to 78% for a moderately effective vaccine to suppress another major outbreak. Also note that natural immunity was taken into consideration and assumed to be protective.
Without a vaccine (scenario 1), the spread of COVID-19 could be suppressed in these states by maintaining strict social distancing measures and face mask use levels. But relaxing social distancing restrictions to the pre-pandemic level without changing the current face mask use would lead to a new COVID-19 outbreak, resulting in 0.8–4 million infections and 15,000–240,000 deaths across these four states over the next 12 months. Under this circumstance, introducing a vaccine (scenario 2) would partially offset this negative impact even if the vaccine effectiveness and coverage are relatively low. However, if face mask use is reduced by 50% (scenario 3), a vaccine that is only 50% effective (weak vaccine) would require coverage of 55–94% to suppress the epidemic in these states. A vaccine that is 80% effective (moderate vaccine) would only require 32–57% coverage to suppress the epidemic. In contrast, if face mask usage stops completely (scenario 4), a weak vaccine would not suppress the epidemic, and further major outbreaks would occur. A moderate vaccine with coverage of 48–78% or a strong vaccine (100% effective) with coverage of 33–58% would be required to suppress the epidemic. Delaying vaccination rollout for 1–2 months would not substantially alter the epidemic trend if the current non-pharmaceutical interventions are maintained.