Fig. 1.

Scenarios for the Course of the Epidemic from 2020 to 2022, for a High-Income Country Setting, in the Absence of a Vaccine (counterfactual scenarios). We assume that R₀ = 2.5 up to time t₁ (May 2020) and that R_t1 drops to 1.1 between time t₁ and t₂ (February 2021). Assuming an average duration of naturally-acquired immunity of one year, this results in 11% in the recovered (immune) state at vaccine introduction. From time t₂ onwards, we consider three counterfactual scenarios, R_t2 = 1.5, 2 and 2.5 shown in light green, purple, and turquoise, respectively. Vaccine impact is compared to these counterfactual scenarios. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)