Oregon Scientists Find Higher Prevalence Of Viral Spread Than Expected

More than 22,600 Oregonians have tested positive for COVID-19 or presumed to have the virus since the start of the pandemic.

But the actual number of infected Oregonians may be over 100,000, or 2.5% of all residents, state health officials said Friday, based on the rate of COVID-19 pathogens revealed in blood testing samples earlier this year.

In 897 blood samples from Oregonians without a coronavirus diagnosis, COVID-19 antibodies were found in nine of them, according to a study co-authored by Dr. Paul Cieslak, medical director of the state’s infectious disease and vaccination program.

Applied to Oregon’s statewide population of about 4.3 million residents, the study would suggest that about 43,000 Oregonians have had symptomatic or asymptomatic cases of COVID-19.

The study, published Friday in the U.S. Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report, analyzed blood samples conducted between May 11 and June 15, right before COVID-19 cases began to spike around Oregon.

“Since then we’ve accrued a lot more cases,” Cieslak said in a Friday briefing. “The estimate of cumulative cases in the state of Oregon is something closer to 2.5%.”

The higher rate would put Oregon’s estimated COVID-19 tally at about 107,500 cases, more than four times the amount of confirmed and presumptive cases Oregon health officials have reported. 

That may be a lower rate than the country overall, which could have as many as 10 times as many cases than have been confirmed, Cieslak said.

“I don’t think we were surprised by this number,” he said. “We know some people are infected without developing symptoms that would lead them to get tested.”

The data suggest a far wider spread of COVID-19 cases around Oregon than has been previously reported, and could mean that hard-hit parts of the state like Umatilla and Malheur counties in Eastern Oregon have even higher prevalence of the coronavirus in their communities.

Gov. Kate Brown on Thursday announced that Malheur County would be downgraded from phase 2 to phase 1 under the state’s economic reopening guidelines, starting Monday.

Her decision came after testing data showed 26% of COVID-19 tests in Malheur County had come back positive over the past two weeks, compared with 5.8% statewide, Oregon Health Authority Director Patrick Allen said Friday.

More than half of the county’s cases over that time couldn’t be traced back to a known source, Allen said.

Brown’s move followed her decision in late July to send Morrow County back to phase 1 and Umatilla County back to baseline pandemic restrictions. At the time, health officials estimated that about 17% of Umatilla County’s roughly 80,000 residents had the virus since the start of the pandemic.

The three rural Eastern Oregon counties – Umatilla, Morrow and Malheur – have had the highest COVID-19 positivity rates in the state for much of the summer.

Health officials said the findings of the blood sample study reinforce how widespread COVID-19 is around the state.

But “it’s also a reminder of how few people in Oregon at this point have really had COVID-19, and how vulnerable our population is to infection,” Allen said.

With 2.5% of the state’s population likely infected since March, the rate of viral spread would have to increase by a factor of 35 to reach the 70% rate necessary to achieve herd immunity, Cieslak said. That leaves a vaccine as the only viable path for health officials to eliminate the spread.

“The major takeaway of the results of the (blood sample) survey is the vast majority of us in Oregon still remain unaffected and susceptible to the disease,” he said.

You can reach Elon Glucklich at [email protected].

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