The effect of a proof-of-vaccination requirement, incentive payments, and employer-based mandates on COVID-19 vaccination rates in New York City: a synthetic-control analysis

BACKGROUND: COVID-19 vaccines have been available to all adults in the USA since April, 2021, but many adults remain unvaccinated. We aimed to assess the joint effect of a proof-of-vaccination requirement, incentive payments, and employer-based mandates on rates of adult vaccination in New York City (NYC). METHODS: We constructed a synthetic control group for NYC composed of other counties in the core of large, metropolitan areas in the USA. The vaccination outcomes for NYC were compared against those of the synthetic control group from July 26, 2021, to Nov 1, 2021, to determine the differential effects of the policies. Analyses were conducted on county-level vaccination data reported by the Centers for Disease Control and Prevention. The synthetic control group was constructed by matching on county-level preintervention vaccination outcomes, partisanship, economic attributes, demographics, and metropolitan area population. Statistical inference was conducted using placebo tests for non-treated counties. FINDINGS: The synthetic control group resembled NYC across attributes used in the matching process. The cumulative adult vaccination rate for NYC (in adults aged 18 years or older who received at least one dose of an authorised COVID-19 vaccine) increased from 72·5% to 89·4% (+16·9 percentage points [pp]) during the intervention period, compared with an increase from 72·5% to 83·2% (+10·7 pp) for the synthetic control group, a difference of 6·2 pp (95% CI 1·4–10·7), or 410 201 people (90 966–706 532). Daily vaccinations for NYC were consistently higher than those in the synthetic control group, a pattern that started shortly after the start of the intervention period. INTERPRETATION: The combination of a proof-of-vaccination requirement, incentive payments, and vaccine mandates increased vaccination rates among adults in NYC compared with jurisdictions that did not use the same measures. Whether the impact of these measures occurred by inducing more people to get vaccinated, or by accelerating vaccinations that would have occurred later, the increase in vaccination rates likely averted illness and death. FUNDING: None.