Epidemiological Impact of SARS-CoV-2 Vaccination: Mathematical Modeling Analyses

Abstract

This study aims to inform SARS-CoV-2 vaccine development/licensure/decision-making/implementation, using mathematical modeling, by determining key preferred vaccine product characteristics and associated population-level impacts of a vaccine eliciting long-term protection. A prophylactic vaccine with efficacy against acquisition (VE_S) ≥70% can eliminate the infection. A vaccine with VE_S <70% may still control the infection if it reduces infectiousness or infection duration among those vaccinated who acquire the infection, if it is supplemented with <20% reduction in contact rate, or if it is complemented with herd-immunity. At VE_S of 50%, the number of vaccinated persons needed to avert one infection is 2.4, and the number is 25.5 to avert one severe disease case, 33.2 to avert one critical disease case, and 65.1 to avert one death. The probability of a major outbreak is zero at VE_S ≥70% regardless of the number of virus introductions. However, an increase in social contact rate among those vaccinated (behavior compensation) can undermine vaccine impact. In addition to the reduction in infection acquisition, developers should assess the natural history and disease progression outcomes when evaluating vaccine impact.

Keywords: COVID-19; SARS-CoV-2; coronavirus; epidemiology; mathematical model; vaccine.

Figure 1

Impact of SARS-CoV-2 vaccination on the number of (A) new infections, (B) new severe disease cases, (C) new critical disease cases, and (D) new deaths in the scenario assuming vaccine scale-up to 80% coverage before epidemic onset. The duration of vaccine protection is 10 years. Impact was assessed at $V{E}_S=50\%$, $V{E}_I=50\%$, $V{E}_{P_1}=50\%$, $V{E}_{P_2}=50\%$, $V{E}_S=V{E}_I=V{E}_{P_1}=50\%$.

Figure 2

Impact of SARS-CoV-2 vaccination on the number of (A) new infections, (B) new severe disease cases, (C) new critical disease cases, and (D) new deaths in the scenario assuming vaccine introduction during the exponential growth phase of the epidemic, with scale-up to 80% coverage within one month. Duration of vaccine protection is 10 years. Impact was assessed at $V{E}_S=50\%$, $V{E}_I=50\%$, $V{E}_{P_1}=50\%$, $V{E}_{P_2}=50\%$, $V{E}_S=V{E}_I=V{E}_{P_1}=50\%$.

Figure 3

SARS-CoV-2 vaccine effectiveness. Number of vaccinated persons needed to avert (A) one infection, (B) one severe disease case, (C) one critical disease case, and (D) one death, by the end of the epidemic cycle, that is, after the epidemic has reached its peak and declined to a negligible level. The scenario assumes vaccine scale-up to 80% coverage before epidemic onset. Duration of vaccine protection is 10 years. Impact was assessed at $V{E}_S=50\%$, $V{E}_I=50\%$, $V{E}_{P_1}=50\%$, $V{E}_{P_2}=50\%$, $V{E}_S=V{E}_I=V{E}_{P_1}=50\%$. Panel A does not include the result for $V{E}_{P_2}=50\%$, as this efficacy has no impact on the number of infections—it affects only severe and critical disease and death.

Figure 4

Effectiveness of age-group prioritization using a SARS-CoV-2 vaccine with VE_s of 50%. Number of vaccinated persons needed to avert (A) one infection, (B) one severe disease case, (C) one critical disease case, and (D) one death by prioritizing different age groups for vaccination. Scenario assumes vaccine scale-up to 80% coverage before epidemic onset and duration of vaccine protection of 10 years. Effectiveness is assessed at the end of the epidemic cycle, that is, after the epidemic has reached its peak and declined to a negligible level.

Figure 5

Impact of varying levels of vaccine efficacy in reducing susceptibility (VE_s) on (A) cumulative number of new SARS-CoV-2 infections (final epidemic size) and (B) number of vaccinated persons needed to avert one SARS-CoV-2 infection. Scenario assumes vaccine scale-up to 80% coverage before epidemic onset. Duration of vaccine protection is 10 years. Measures are assessed at the end of the epidemic cycle, that is, after the epidemic has reached its peak and declined to a negligible level.

Figure 6

Impact of vaccination with reduced adherence to social distancing for those vaccinated. Figure shows the impact of varying levels of behavior compensation post-vaccination on the vaccine-induced reduction in the cumulative number of new SARS-CoV-2 infections by the end of the epidemic cycle. Scenario assumes vaccine scale-up to 80% coverage before epidemic onset, VE_s is 50%, and duration of vaccine protection is 10 years.

Figure 7

Probability of occurrence of a major outbreak following vaccination. Probability of occurrence of a major outbreak upon virus introduction at varying levels of (A) $V{E}_S$, (B) $V{E}_I$, (C) $V{E}_{P_1}$, and (D) $V{E}_S=V{E}_I=V{E}_{P_1}$. Scenario assumes vaccine scale-up to 80% coverage before epidemic onset. Duration of vaccine protection is 10 years. The figure does not include the result for $V{E}_{P_2}$, as this efficacy has no impact on the probability of occurrence of a major outbreak. The analysis and derivation for the probability of occurrence of a major outbreak can be found in Text S1D of the Supplementary Material.