SALEM — The number of Oregonians getting vaccinated against COVID-19 has nose-dived since Gov. Kate Brown announced a $1 million lottery for vaccinated residents a few weeks ago.

The state says it’s now looking at ways to spark more enthusiasm — and inoculations among those still unvaccinated — in the three weeks left before the drawing is held.

Charles Boyle, a governor’s spokesman, said in an email that the governor’s office is “exploring options for drawings earlier in the month to continue to generate excitement throughout the next several weeks.”

Brown also hinted at a news conference Friday that more prizes may be added to the offerings in coming days, though she hasn’t yet shared any details.

“We have more surprises in the works that you’ll hear about soon,” Brown said. “So if you haven’t yet been vaccinated, now is the time. You never know, you may just walk into a vaccine clinic or a pharmacy to get your vaccine and find out you’re a winner.”

Oregon is among one of at least nine states — including Washington last week — drumming up lotteries in hopes of elevating immunization numbers that have plummeted in the late spring nationwide. Most are offering to make at least one person in each of their borders a millionaire — but only if they’re vaccinated against COVID-19.

New Mexico has taken it a step further by putting up the largest-in-the-nation prize: $5 million.

Brown announced May 21 that Oregon is offering a $1 million jackpot, three dozen awards of $10,000 doled out to one person in each of the state’s counties, and five scholarships of $100,000 for youth for college or vocational school.

But unlike other states that started their drawings within a week or two of their announcements and that are holding drawings weekly or even daily, Oregon will draw all its winners on one day, June 28, which is more than five weeks after Brown announced the lottery’s creation.

Officials estimate it could be another week beyond that date before they announce the winners.

State officials seem to have acknowledged that delayed timeline might have put a damper on the initial zeal and urgency to go out and get vaccinated as soon as possible.

“The ‘Take Your Shot Oregon’ campaign was designed for simplicity, so that there would be a significant period of time to incentivize vaccinations prior to June 28,” said Boyle, before adding that the state might tweak that strategy to award some new prizes soon.

The lottery, he cautioned, is one of many strategies the state is using to increase inoculations.

“No individual strategy is expected to have a singularly massive impact or to wholly reverse vaccination rate trends,” Boyle said. “Each strategy adds a little energy to the overall effort.”

So far, the day-to-day number of newly vaccinated people in Oregon has been disappointing.

While Oregon was administering about 17,000 shots as of midweek last week — about half of the number it was administering when Brown announced the lottery — that includes shots for people who already opted to get vaccinated and are receiving a second dose.

The key metric to watch to gauge the lottery’s performance is the number of unvaccinated people receiving their first shots.

The seven-day average of adults receiving their first shots has gone from about 9,000 the day before Brown announced the lottery to 6,700 nearly two weeks later. (Because state officials say it can take up to five days for vaccinations to be entered into the system, The Oregonian did not use the last five days of data.)

The decline in newly vaccinated people stands at about 25%. And that drop-off means it’s taking longer for Oregon to hit a separate benchmark established by Brown to lift nearly all coronavirus restrictions, including wearing masks in most settings.

States across the country launched similar lotteries in the face of declines, and none has yet seen stellar results.

Ohio was the first state to announce a lottery, on May 12, and Gov. Mike DeWine later hailed the effort as a success.

Data from the Centers for Disease Control and Prevention and analyzed by The Oregonian show initial stability compared to Oregon.

Ohio had been newly vaccinating about 19,500 adults, on average, the day before the lottery announcement. Two weeks later, the average number of people being newly vaccinated had fallen only 4%, although it’s since declined further.

The newspaper reviewed vaccination data for other states with lotteries, tracking the number of newly vaccinated adults and also accounting for the shrinking pool of residents in need of inoculation.

Maryland, with its daily drawings, and New York, with its weekly drawings, appear to have held relatively steady overall and even made some very modest gains, after factoring in fewer people needing shots.

California saw a slight dip in vaccination in the first several days after Gov. Gavin Newsom announced the state’s lottery but also saw a spike in residents getting vaccinated this past weekend immediately after Newsom drew the first 15 $50,000 winners Friday on livestreamed video. The state recorded more than 115,000 newly vaccinated people Saturday, the highest daily total in nearly a month.

Dr. A. Mark Fendrick, a public health professor and director of the University of Michigan Center for Value-Based Design who has studied what incentives or disincentives motivate people to make smart choices in their health care, said it’s too early to tell how much this vast experiment of COVID-19-vaccination lotteries influenced individual decision-making. He will be studying the state-by-state data for months to come, including how the duration between drawings and the amounts of prize money offered affected human behavior.

“What we’re really happy about is that the next time around that we need to do this, we’re going to know the best way to do it,” Fendrick said. “We have this great natural experiment of no lotteries versus lotteries and types of lotteries versus other types of lotteries.”

Fendrick said if the lotteries do turn out to be a significant influence, it could help the health care system figure out how to better motivate pregnant women to get prenatal care of certain groups to get screened for breast, cervical or colon cancers.

David Baden, chief financial officer at the Oregon Health Authority, said even though vaccination rates have decelerated, there is some data to suggest that the state’s “Take Your Shot” campaign might be having some positive effect: Oregon’s vaccination rates aren’t decelerating as swiftly as other states, boosting the state’s rankings.

A few weeks ago, Oregon was 23rd nationally for the percentage of its population at least partially vaccinated, Baden said. On Monday, it stood at 17th.

The end result? Baden said because there’s still a few weeks left before the June 28 drawing, he’s holding off on reaching conclusions about the lottery’s success.

“The simple answer is, ‘I don’t know because we don’t ask everyone who comes in,’” Baden said, “‘Why did you get vaccinated?’”