Nearly one in five Oregonians infected with COVID-19 in July was fully vaccinated against the disease.

The news represents a sizable leap in the share of total infections involving fully vaccinated people — from 2% in April to 19% in July — and underscores emerging information that the highly contagious delta variant is more likely to infect vaccinated people than earlier versions of the virus.

In a breakthrough cases report posted to the Oregon Health Authority’s website Thursday, officials say about 2,400 of the 12,514 people known to be infected by COVID-19 in the state in July were fully vaccinated. That leaves 81% of the infected Oregonians — or just over 10,000 in July — who were either unvaccinated or only partially vaccinated.

“How worried should people be?” said Dr. Melissa Sutton, medical director of respiratory viral pathogens at the Oregon Health Authority, of breakthrough cases. “I’d say there’s an incredible amount of disease in the community right now. If you’re vaccinated, you are well protected but the vaccines are not perfect.”

Sutton, who is fully inoculated, said she wears a mask in indoor public spaces and even around crowds outdoors. “I sleep better at night knowing that I’m vaccinated and that I’m so unlikely to end up in the hospital. But I do think people should take precautions. Everyone.”

The data in this month’s report highlights the nuanced and daunting challenge public health officials face in their messaging to the public: The vaccines don’t offer 100% protection against getting infected, but getting fully vaccinated dramatically reduces the likelihood of serious illness and death. So the wise choice, officials say, is to join the 2.3 million Oregonians and 166 million Americans who already are fully inoculated against the coronavirus.

“They are not the ones in the hospital, they’re not the ones going on ventilators and they’re not the ones who are dying from this,” with very few exceptions, said Joe Fiumara, public health director of Umatilla County, which has one of Oregon’s lowest vaccination rates and highest infection rates.

Some people will use the latest breakthrough numbers as an illogical justification to shun vaccines, experts say.

“The problem is many people are using this to reinforce the stance they already wanted to take, which is, ‘It’s not going to matter. I’m still going to get sick if I get the vaccine,’” Fiumara said.

According to the Oregon Health Authority, the rate of unvaccinated or partially vaccinated people getting infected is nearly six times the rate of fully vaccinated people in the last week of July. That’s roughly about 265 unvaccinated or partially vaccinated people infected per 100,000 residents, compared to roughly 45 fully vaccinated people per 100,000 residents, according to graphics included in the report.

While the report didn’t give specifics for July, it states that more than 90% of Oregonians with COVID-19 who are hospitalized within two weeks of their positive tests weren’t fully vaccinated. That’s consistent with reports from local hospitals. The percentage of unvaccinated hospitalized COVID-19 patients at Samaritan Health Services, which operates five hospitals in Oregon, doesn’t drop below 95%, said chief physician executive Dr. Robert Turngren.

In July, 91% of COVID-19 deaths in Oregon were among people who were unvaccinated or partially vaccinated, according to the report. That’s 50 of 55 deaths.

While the state didn’t share specific details for each of those cases, officials say of the 42 fully vaccinated people who’ve died in Oregon of COVID-19 since vaccinations began nearly eight months ago, the median age is 83.

The median age of fully vaccinated Oregonians who became infected was 51. Older people are more susceptible to breakthrough cases: While people over age 65 comprise 18% of the state’s population, they’ve accounted for 27% of breakthrough infections. About 7% of cases have been in congregate care settings, such as nursing homes.

One reason experts say fully vaccinated people might be testing positive in higher numbers than in previous months is that the delta variant is so much more contagious than previous dominant strains that it’s more effective at infecting everyone. A much higher percentage of the population also is fully vaccinated now, and so based on sheer numbers of whom the virus encounters, fully vaccinated people are more likely to comprise a larger percentage of cases if they’re exposed to extreme viral loads in enclosed spaces.

Dr. Sarah Present, Clackamas County health officer, said she thinks both factors are at play: One, the delta variant is proving to be a more formidable foe, reinforcing the fact that the vaccines aren’t bulletproof. Two, breakthrough cases are naturally increasing because more people are inoculated.

Last week, 94 of Clackamas County’s 404 diagnosed COVID-19 cases — or 23% — were in fully vaccinated people, she said.

“I think we all felt like cases were coming down and we might see an end to this pandemic, and instead we are seeing a huge surge that could be worse than what we saw in the fall or spring,” Present said. “It’s distressing and it’s disturbing.”

Present stressed, however, that the fully vaccinated should feel far better about their prospects because they’re far more protected against severe disease and death than those who haven’t received their doses.

In their report, state public health officials also stressed the bigger picture: “Although the number of vaccine breakthrough cases is increasing, they are very small when compared to the more than 2.3 million people who have completed their COVID-19 vaccination series.”

Within a matter of two months, the delta variant has become responsible for virtually all COVID-19 cases in Oregon and across the nation. Scientists say it’s more crucial than ever for all eligible Americans to get fully vaccinated because the full two-dose regimen of Pfizer-BioNTech and Moderna vaccines offers much greater protection against the delta variant than just one dose. One recent study found Pfizer-BioNTech’s vaccine is 88% effective at preventing symptomatic disease against delta, and it’s thought to be even higher for severe disease and death. Moderna is thought to be similarly effective because it’s also an mRNA vaccine.

There’s been more debate about the effectiveness of Johnson & Johnson’s vaccine. A new study shows it was 71% effective against hospitalization and 95% against death against the delta variant in a South African trial.

Sutton, the Oregon Health Authority medical director, said the latest breakthrough report doesn’t specify the type of vaccine received by fully vaccinated people who became infected because of a shortage of staff to crunch the data. Sutton said she can say that among the pool of fully vaccinated people sickened by COVID-19, Johnson & Johnson recipients appear to be making up a larger percentage of those hospitalized.

Oregon has been publishing its breakthrough cases report once a month, but on Friday Sutton said the state would start publishing the report every two weeks.

Read the latest report here.

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— Aimee Green; agreen@oregonian.com; @o_aimee