The results of the Philly Vax Sweepstakes are in: It was no shot in the arm for the city’s stagnating COVID-19 vaccination rates earlier this summer, not even in neighborhoods where residents were more likely to win cash prizes up to $50,000.

The University of Pennsylvania researchers who designed, funded, and studied the sweepstakes concluded in a draft research paper that offering jackpots doesn’t work. Policy makers, they wrote, would be better off looking for “more impactful ways to encourage vaccination.”

Mayor Jim Kenney, who announced the inducements in early June with a can’t-hurt-might-help attitude, was equally philosophical about the results.

“While it’s disappointing that the sweepstakes didn’t do more to encourage vaccinations, I’m so glad that this program and other incentives have motivated some people to get vaccinated,” Kenney said in an email. “We will continue to look for ways to encourage residents to join the more than 808,000 Philadelphians who are fully vaccinated. Whatever your motivation is, please know that vaccines are the best and safest way we can get through this deadly and unpredictable pandemic.”

Across the country, state and local governments have launched vaccine lotteries — some with six- or even seven-figure grand prizes — to entice the hesitant to roll up their sleeves. Vaccination of the vast majority of a population is the only hope for snuffing out the pandemic, public health experts agree.

The effectiveness of such lotteries has been mixed, at best, and often hard to interpret. For example, Ohio has called its Vax-A-Million program a “resounding success,” and two research teams concurred — but two other teams concluded just the opposite.

Proving a lottery-related uptick in vaccinations is tricky, especially if nearby jurisdictions without lotteries also see upward trends. Other factors such as targeted vaccine clinics or door-to-door outreach — or soon, vaccine mandates — may be the driving forces.

To get more precise data, the Penn researchers added an experimental component to the Philly Vax Sweepstakes, basically stacking the deck in favor of residents in three, randomly chosen zip codes with high poverty rates and low vaccination rates. Half of the winners were drawn from those zip codes alone, with the rest of the winners selected from elsewhere in the city.

That gave people in the low-vaccination zip codes about 73 times better chances of winning — as long as they had already gotten at least one shot.

The stakes were considerable: six $1,000 prizes, four $5,000 prizes, and two $50,000 prizes were awarded in each of the three drawings. In theory, vaccine holdouts would imagine getting a call telling them they had won, then be disqualified because they were unvaccinated. That would induce them to get vaccinated pronto to avoid such a regrettable situation.

So-called regret lotteries have been used to motivate weight loss, exercise, and sticking to medical treatments, but most of the studies have been small, noted the Penn researchers, led by Wharton School professor Katherine Milkman.

Because the success of the Philly incentive hinged on unvaccinated residents being aware of their potential windfall, the sweepstakes was heavily publicized and promoted. The Inquirer, other newspapers, and broadcast outlets all ran stories. Residents were encouraged to actively register for the program online or by phone, although the researchers also had a citywide commercial database.

Ultimately, the citywide results showed a slight initial bump in vaccinations in Philly compared to surrounding counties, but it wasn’t sustained.

Of the three low-vaccination zip codes, only one had an brief uptick compared to 17 “control” zip codes.

“When pooling our data across the three zip codes, we do not detect any significant change,” wrote the Penn researchers.

When the sweepstakes was launched, about 67% of Philadelphia adults had received at least one dose of vaccine, according to city data. But disparities were obvious. In the three under-vaccinated zip codes, the rates were under 40%.

Now, city data shows about 80% of Philadelphians have had at least one shot, but five zip codes still had less than 40% of their residents fully or partially vaccinated early this month — rates on par with Louisiana and Arkansas.