FIGURE 1.

Imputation of strain specific cases for a counterfactual placebo group in period 2. Imputation #1 of 5 ancestral (green) cases follows from portability of ancestral VE of 0.80 for the newly vaccinated which is estimated in period 1. Imputation # 2 of 10 variant (red) cases follows from the 2:1 variant:ancestral case split seen in the surveillance cohort. With ancestral and variant placebo case counts, early and late vaccine efficacy for ancestral and variant strains is easy to calculate. For example, early vaccine efficacy for the variant strain (VE${}_{11}$) is estimated as $1-4/10=0.60$. Using the notation of Table 1, we have period 1 counts of $X_{110}=2,X_{010}=10$, and period 2 counts of $X_{121}=5,X_{120}=3,X_{021}=4,X_{020}=1$