Projected COVID-19 epidemic in the United States in the context of the effectiveness of a potential vaccine and implications for social distancing and face mask use

Abstract

Background: Multiple candidates of COVID-19 vaccines have entered Phase III clinical trials in the United States (US). There is growing optimism that social distancing restrictions and face mask requirements could be eased with widespread vaccine adoption soon.

Methods: We developed a dynamic compartmental model of COVID-19 transmission for the four most severely affected states (New York, Texas, Florida, and California). We evaluated the vaccine effectiveness and coverage required to suppress the COVID-19 epidemic in scenarios when social contact was to return to pre-pandemic levels and face mask use was reduced. Daily and cumulative COVID-19 infection and death cases from 26th January to 15th September 2020 were obtained from the Johns Hopkins University Coronavirus resource center and used for model calibration.

Results: 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.

Conclusions: The degree to which the US population can relax social distancing restrictions and face mask use will depend greatly on the effectiveness and coverage of a potential COVID-19 vaccine if future epidemics are to be prevented. Only a highly effective vaccine will enable the US population to return to life as it was before the pandemic.

Keywords: COVID-19 vaccine; Face mask use; Social distancing; Vaccine coverage; Vaccine effectiveness.

Fig. 1

Flow chart of COVID-19 transmission model. The total population is divided into ten compartments (susceptible individuals (S), vaccinated individuals (V), latent infections (E), asymptomatic infections (A), undiagnosed infections with mild/moderate (I₁) and severe/critical symptoms (I₂), diagnosed infections with mild/moderate (T₁) and severe/critical symptoms (T₂), and recovered (R) and deceased (D) cases). The force of infection for susceptible and vaccinated individuals are denoted as $\mathrm{\Lambda }$ and ${\mathrm{\Lambda }}_V$, which involves two transmission patterns: public settings (e.g., public transportation, supermarkets, offices, etc.) and households. The model includes three control measures: handwashing, social distancing and face mask use. More details on $\mathrm{\Lambda }$ and ${\mathrm{\Lambda }}_V$ are provided in the Supplementary Appendix. The average incubation period is 1/k₁, and the probability that an individual is asymptomatic is $\rho$. Infectious individuals with mild/moderate and severe/critical symptoms are diagnosed and treated at the rates ${\alpha }_1$ and ${\alpha }_2$, respectively. We assume these diagnosed individuals are isolated strictly and could not further infect others. Undiagnosed and diagnosed mild/moderate cases progress to the severe/critical stage at the rates $k_2$ and $k_3$, respectively. Asymptomatic infections and undiagnosed mild/moderate cases are assumed to recover naturally at the rate ${\gamma }_0$. Diagnosed mild/moderate and severe/critical cases will recover at the rates ${\gamma }_1$ and ${\gamma }_2$, respectively. Undiagnosed and diagnosed severe/critical cases will die due to the disease at the rates ${\mu }_1$ and ${\mu }_2$, respectively. The vaccination rate is denoted as w. Social distancing restrictions relaxed in the scenarios in Methods means that the public person-to-person contact rates m₁(t) in $\mathrm{\Lambda }$ and ${\mathrm{\Lambda }}_V$ (see Supplementary Appendix) recovered to 100% of the pre-pandemic level.

Fig. 2

Model calibration and data fitting based on reported confirmed COVID-19 cases and deaths in four states, i.e., New York (a-b), Texas (c-d), Florida (e-f), and California (g-h). The blue areas denote 95% confidence intervals. Dashed lines, dash-dot lines, and dotted lines denote the social distancing order (public person-to-person contact rates decreased), face mask order, and reopening (public person-to-person contact rates recovered to no more than 100% of the pre-pandemic level) policies that were implemented in each state, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Contour plots of averted infections as a function of vaccine effectiveness and vaccine coverage rate in four states when social distancing restrictions are relaxed to pre-pandemic level shortly after the commencement of vaccination, and maintaining face mask use to the baseline level (the first row, i.e., scenario 2 in Methods), half of the baseline level (the second row, i.e., scenario 3 in Methods), and no use (the third row, i.e., scenario 4 in Methods), compared with no vaccine (scenario 1 in Methods). The black solid isoclines indicate the threshold that the number of averted infections is zero. The black dashed lines correspond to the minimal vaccine effectiveness and vaccine coverage rate when the number of averted infections is zero.

Fig. 4

Contour plots of averted deaths as a function of vaccine effectiveness and vaccine coverage rate in four states when social distancing restrictions are relaxed to pre-pandemic level shortly after the commencement of vaccination, and maintaining face mask use to the baseline level (the first row, i.e., scenario 2 in Methods), half of the baseline level (the second row, i.e., scenario 3 in Methods), and no use (the third row, i.e., scenario 4 in Methods), compared with no vaccine (scenario 1 in Methods). The black solid isoclines indicate the threshold that the number of averted deaths is zero. The black dashed lines correspond to the minimal vaccine effectiveness and vaccine coverage rate when the number of averted deaths is zero.