Epidemiology and control of SARS-CoV-2 epidemics in partially vaccinated populations: a modeling study applied to France

Abstract

Background: Vaccination is expected to change the epidemiology and management of SARS-CoV-2 epidemics.

Methods: We used an age-stratified compartmental model calibrated to French data to anticipate these changes and determine implications for the control of an autumn epidemic. We assumed vaccines reduce the risk of hospitalization, infection, and transmission if infected by 95%, 60%, and 50%, respectively.

Results: In our baseline scenario characterized by basic reproduction number R₀=5 and a vaccine coverage of 70-80-90% among 12-17, 18-59, and ≥ 60 years old, important stress on healthcare is expected in the absence of measures. Unvaccinated adults ≥60 years old represent 3% of the population but 43% of hospitalizations. Given limited vaccine coverage, children aged 0-17 years old represent a third of infections and are responsible for almost half of transmissions. Unvaccinated individuals have a disproportionate contribution to transmission so that measures targeting them may help maximize epidemic control while minimizing costs for society compared to non-targeted approaches. Of all the interventions considered including repeated testing and non-pharmaceutical measures, vaccination of the unvaccinated is the most effective.

Conclusions: With the Delta variant, vaccinated individuals are well protected against hospitalization but remain at risk of infection and should therefore apply protective behaviors (e.g., mask-wearing). Targeting non-vaccinated individuals may maximize epidemic control while minimizing costs for society. Vaccinating children protects them from the deleterious effects of non-pharmaceutical measures. Control strategies should account for the changing SARS-CoV-2 epidemiology.

Keywords: Non-pharmaceutical interventions; SARS-CoV-2; Vaccination.

Fig. 1

Contribution of groups defined by their age and vaccination status to infections, disease spread and hospital burden, in our baseline scenario with R₀=5 and a vaccine coverage of 70–80–90% among 12–17 years old, 18–59 years old, and over 60 years old. Age distribution of new infections (A) in the entire population and (B) among vaccinated and unvaccinated individuals. Proportion of infections (C) attributable to different age groups and (D) attributable to different age groups among vaccinated and unvaccinated individuals. Age distribution of hospitalizations (E) in the entire population and (F) among vaccinated and unvaccinated individuals. In all panels, the diamonds indicate the age distribution of the different groups in the population

Fig. 2

Comparison of the impact of control strategies targeting the entire population vs unvaccinated individuals only, in our baseline scenario with R₀=5 and a vaccine coverage of 70–80–90% among 12–17 years old, 18–59 years old, and over 60 years old. (A) Peak in daily hospital admissions under different testing strategies. Baseline, no intervention; Autotest unvaccinated, 50% of the unvaccinated individuals aged ≥12 years old are tested weekly (sensitivity of 75%); Autotest random, the same number of individuals as in the Autotest unvaccinated are tested but among individuals aged ≥12 years old, irrespective of vaccine status; Antigenic unvaccinated, same as in Autotest unvaccinated but with tests performed by a professional (sensitivity of 90%); Antigenic random, same as in Autotest random but with tests performed by a professional (sensitivity of 90%); Vaccinate, 50% of the unvaccinated individuals aged ≥12 years old are vaccinated. B Peak in daily hospital admissions under non-pharmaceutical interventions of varying intensities. Baseline, no intervention; Reduction of x% unvaccinated, The transmission rate of unvaccinated individuals is reduced by x%; Reduction of x% all, The transmission rate at the population level is reduced by x%. We assume 25% of the population has acquired protection through natural infection (range 20–30% corresponding to the vertical bars)

Fig. 3

Expected size of the peak of hospitalizations when non-pharmaceutical interventions target unvaccinated individuals only or the whole population, as a function of the basic reproduction number R₀, vaccine coverage in the 12–17 years old, 18–59 years old, and over 60 years old and for different efficacy of the vaccine against the risk of infection or hospitalization. Non-pharmaceutical interventions reduce the transmission rate of unvaccinated individuals (points) or the whole population (triangles) by 0%, 10%, 20%, 30%, and 40%. R₀ takes the values 3.0 (top row, A, B, C), 4.0 (D, E, F), 5.0 (G, H, I), and 6.0 (bottom row, J, K, L). In the baseline scenario (left column) we assume that the vaccines are 95% effective at reducing the risk of hospitalization, 60% at reducing the risk of infection, and 50% at reducing the infectivity of vaccinated individuals. In sensitivity analyses, we consider an 80% reduction against infection (middle column) and a 90% reduction against hospitalization (right column). We assume 25% of the population has acquired protection through natural infection (range 20–30% corresponding to the vertical bars). Horizontal lines indicate the peak of daily hospital admissions observed during the first (dashed line) and the second (dotted line) epidemic wave of 2020

Fig. 4

Proportion of infections (A, C, E) and hospitalizations (B, D, F) among groups defined by their age and vaccination status as a function of the vaccine coverage in the 12–17 years old, 18–59 years old, and over 60 years old. In the baseline scenario (A, B), we assume that vaccines are 95% effective at reducing the risk of hospitalization, 60% at reducing the risk of infection and 50% at reducing the infectivity of vaccinated individuals. In (C and D), we assume a vaccine efficacy at reducing the risk of infection of 80%. In (E and F), we assume a vaccine efficacy at reducing the risk of hospitalization of 90%. The distribution is reported for infections and hospitalizations occurring between September 1st, 2021, and March 20th, 2022 (end of the study period), for R₀=5.0. We assume 25% of the population has acquired protection through natural infection