Gov. Jay Inslee has claimed Washington’s vaccine lottery led to a 24% increase in vaccination rates statewide. “The ‘Shot of a Lifetime’ is working & more people are getting vaccinated,” he said on Twitter earlier this month.

There’s been mixed results across the country. Some states that implemented lotteries have reported boosts in vaccinations. But Ohio, the first state to offer a cash-prize incentives and rewards, saw an early spike slow and has ended the program. 

A data analysis by Politico in four other states found lotteries “haven’t reversed the steep decline in adults seeking out shots.” And in Washington, not all prizes have been claimed, posing the question of how many vaccinated residents even know they’ve been entered into a lottery. The state’s $1 million winner said on Friday, he didn’t know about the state’s incentive program when he received the call notifying him he had won.

Inslee said the Institute for Disease Modeling estimated Washington’s lottery has so far led nearly 30,000 additional people to get vaccinated against the coronavirus.

How did the state figure that out? The 30,000 estimate does not represent a 24% increase in the total number of residents vaccinated. Instead, it is based on the comparison of the number of people who were expected to get vaccines during this period of falling vaccination rates, and the number who actually ended up getting shots.

Franji Mayes, an emergency communications consultant for the health department, explained how the state got its numbers.

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To estimate the effect of the “Shot of a Lifetime” promotion on vaccination rates, the state started by looking at the daily percentage of unvaccinated people who received their first shot by age, since vaccine eligibility and trends differ by age group.

In general, they had seen that once an age group becomes eligible for the vaccine, vaccination rates increase rapidly, hit a peak, then decline steeply. By the end of May, right before the lottery was announced, vaccination rates for people getting their first dose were decreasing on average by about 3 to 10% each day, Mayes said.

In particular, younger age groups were seeing higher rates of decline.

After the announcement, the state saw an increase in vaccination rates across younger age groups, a steady hold for middle age groups and decreased rates of decline in older age groups, she said.

“Since rates were already declining fast before the announcement and we expected those declines to continue, we wouldn’t necessarily see the impact of the lottery if we just look for an increase in overall vaccination rates,” Mayes said. “To understand the likely impact, we need to estimate the vaccination rates we would have expected to see if the lottery hadn’t happened and compare them with the rates we actually saw.”

When they compared the rates projected for each age group without the lottery to the rates seen once the lottery was implemented, they found:

  • The largest difference between the expected rates and actual rates occurred in those under 18, where actual rates increased and pre-lottery trends were decreasing the fastest.
  • The differences between the expected rates and actual rates were progressively smaller for older age groups, with minimal difference in the oldest age groups.

“Then, to get an idea of the overall effect of the lottery, we put together the estimates for how many additional people 16 and older got their first shot,” she said. “Our best estimate is that between June 3 and June 22, about 28,500 more people got their first shot than we would have expected without the lottery — a 24% increase.”

A separate lottery for residents who were vaccinated through Veterans Affairs or the Department of Defense will be drawn on July 20.