Fig. 2.

Optimization of the delay between the first and second doses of a COVID-19 vaccine based on first-dose efficacy and vaccine mechanism. The shaded areas represent the first-dose efficacy that results in doubling the time to second dose from the baseline (4 wk). Left shows the second-dose delay when the vaccine alleviates symptoms; in this case, the best strategy delays the second dose for $\ge$8 wk when the first-dose efficacy is $\ge$70%. Right shows the second-dose delay when the vaccine blocks infection; here, the best strategy delays the second dose for $\ge$8 wk when the first-dose efficacy is $\ge$50%. For both vaccine types, the second-dose efficacy reaches 82.4%. The filled circles show the time to the second dose for $r_0=2.5$, and the bars represent the variability across simulations when $r_0$ is varied from 1.8 to 3 in 0.2 steps.