Table 2. Multivariable Analysis of Vaccination Coverage and Associated Risk Reduction in COVID-19 Deaths.

Wave	Factor	IRR (95% CI)a	P value	aIRR (95% CI)a	P value
Alpha	Week	0.90 (0.88-0.91)	<.001	0.90 (0.88-0.91)	<.001
	Wave	11.1 (6.47-19.1)	<.001	10.9 (1.64-2.70)	<.001
	Wave × vaccination coverageb	0.61 (0.51-0.72)	<.001	0.61 (0.52-0.72)	<.001
Delta	Week	0.94 (0.92-0.96)	<.001	0.94 (0.92-0.96)	<.001
	Wave	13.8 (5.88-32.3)	<.001	13.8 (6.05-31.3)	<.001
	Wave × vaccination coverageb	0.76 (0.65-0.88)	<.001	0.76 (0.66-0.87)	<.001

Abbreviations: aIRR, adjusted incidence rate ratio; IRR, incidence rate ratio.

^a Estimates of IRRs were obtained from a mixed-effects Poisson generalized linear model with log link using fixed effects (wave and the association between wave and vaccination coverage) and random effects of zip code on intercept. aIRRs were estimated from a second model which included percentage of population older than 65 years and percentage of population previously recovered from COVID-19 as fixed effects. The IRR for week was calculated for a 1-week change. The wave IRR was calculated with an indicator variable for after the beginning of each wave (March 28, 2021, for Alpha and August 1, 2021, for Delta). CIs and P values were obtained from asymptotic Wald tests. Full model output is reported in eTable 4 (Alpha wave) and eTable 5 (Delta wave) in the Supplement.

^b For vaccination coverage, a 1-unit increase corresponds to a 10–percentage point increase in people with at least 1 dose.