Guaranteed small cash incentives were widely employed by policy makers during the COVID-19 vaccination campaign, but the impact of these programs has been largely understudied. We were the first to exploit a statewide natural experiment of one such program implemented in West Virginia in 2021 that provided a $100 incentive to fully vaccinated adults ages 16-35. Using individual-level data from the Census Bureau's Household Pulse Survey, we isolated the policy effect through a difference-in-discontinuities design that exploited the discontinuity in incentive eligibility at age thirty-five. We found that the $100 incentive was associated with a robust increase in the proportion of people ever vaccinated against COVID-19 and the proportion who completed or intended to complete the primary series of COVID-19 vaccines. The policy effects were also likely to be more pronounced among people with low incomes, those who were unemployed, and those with no prior COVID-19 infection. The guaranteed cash incentive program may have created more equitable access to vaccines for disadvantaged populations. Additional outreach may also be needed, especially to unvaccinated people with prior COVID-19 infections.