Some state-run vaccine lotteries did not help increase COVID-19 immunization rates, a new study suggests.

Over the spring and summer, at least 19 states — including California, New York, Ohio and West Virginia — tried to incentivize unvaccinated individuals to get shots, offering cash prizes, free tickets, guns, college scholarships and trucks.

However, research from the Boston University School of Medicine found some of these prizes had little to no effect on convincing residents to get vaccinated against COVID.

For the study, published in JAMA Internal Medicine, the team compared vaccination rates between 15 states that offered lotteries with cash prizes and 31 states that did not between May 24, 2021 and July 19, 2021.

Data of daily rates of first doses came from the U.S. Centers for Disease Control and Prevention.

The study found that about four weeks before the lottery announcement, the lottery states were vaccinating an average of 225 per 100,000 people with their first doses.

Immediately after the lottery announcement, the rate increased by 1.1 per 100,000 people.

However, by the fourth week following the lottery, the rate had fallen to fewer than 100 per 100,000 people receiving their first shots.

The trend was similar among U.S. states without lotteries, which experienced a decline in rates nearly mirroring those seen in the lottery states.

Vaccine lotteries had initially been deemed a success after reports that vaccination rates had significantly boosted, such as in Ohio, where officials said they saw a 55% increase in vaccinations for young adults following the state’s Vax-A-Million lottery.

However, it appears the boosts were likely temporary.

One limitation of the study is the small number of states analyzed. Because researchers only looked at 15 states with lotteries, small increases in vaccination rates may not have been detected.

The team insists, however, that the findings are strong and that more research should be conducted on vaccine incentives that work.

“This study did not find evidence that vaccine lottery incentive programs in the U.S. were associated with significantly increased rates of COVID-19 vaccinations,” the researchers wrote.

“Given the lack of a strong association between state lottery-based vaccine incentives and increased vaccination rates, further studies of strategies to increase vaccination rates are needed.”

A previous study from Boston University found similar results when researchers looked at Ohio, specifically comparing vaccination rates from one month before the lottery started — April 15 to May 12 — and one month after the lottery was announced — May 13 to June 9.

They found the daily vaccination rates declined from 485 shots for every 100,000 adults prior to the lottery to 101 for every 100,000 by early June.