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Study Suggests Oregon’s Official COVID-19 Count Is Too Low

Oregon Health & Science University nurse practitioner Shelby Freed pulls a COVID-19 test swab from its sleeve at a drive-up station in Portland, Ore., Friday, March 20, 2020.
Bradley W. Parks

An antibody survey conducted by the Oregon Health Authority finds that by June 15th, an estimated 1% of Oregonians had antibodies for the virus that causes COVID-19. That's ten times higher than the official estimate for June.

Oregon’s official COVID-19 case count is far too low to be considered accurate, according to a new study that tested Oregon residents for antibodies for the new coronavirus. The research was conducted by the Oregon Health Authority and published as a Field Note in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.

Since the study period was completed two months ago, the report can’t say much about how many people in Oregon have been exposed to COVID-19 as of today.

”Since then, we’ve accrued a lot of cases,” said Paul Cieslak, a scientist at OHA and an author on the study. “Our current estimates of cumulative cases in the state of Oregon are something closer to 2.5%.”

The study’s relatively high percent of people infected helps confirm the suspicion that a majority of Oregon’s COVID-19 cases are going undetected — due in part to testing shortages, inaccurate tests, asymptomatic cases and people who did not receive medical treatment. As of June 15, about 1% of Oregonians had been exposed to COVID-19. That’s ten times higher than the percentage of Oregonians confirmed to have had COVID-19 by that date: around 0.1%.

Although this is consistent with several other studies across the country show that only about 1 in 10 COVID-19 cases are diagnosed, it is seemingly at odds with OHA’s own modeling. OHA partners with the Institute for Disease Modeling at the University of Washington to create models that it uses to plan and estimate hospital capacity. These models consistently estimated that Oregon’s real case count was 3 to 5 times higher than the confirmed case count, not ten. The most recent model, published Aug. 5, says that while 19,200 COVID-19 cases were identified by July 31, the actual cumulative case count was estimated to be closer to 88,000.

Cieslak said that despite the study’s finding of inaccuracy in Oregon’s official COVID-19 case numbers, it doesn’t go so far as to call OHA’s model into question. All told, 897 Oregonians had their blood tested. Nine of them had antibodies for SARS-CoV-2, the virus that causes COVID-19. Although that’s at right about 1%, the small sample size means that the actual prevalence could have been much lower, or much higher. Cieslak says that once you account for chance and variability, the model estimates are consistent with the potential range of case rates found in this study.

Because the sample size is so small, Cieslak also cautions against using it to determine how many people in Oregon have COVID-19. “It’s just one datapoint.”

In the months since early June, it has become clear that SARS-CoV-2 is not evenly distributed throughout Oregon.

Outbreaks and super-spreading events have occurred in several counties. In a survey conducted July 25-26, Oregon State University’s TRACE study found that 17% of Hermiston, Oregon residents may have had COVID-19 during their study period. In Malheur County, Oregon, the Oregon Health Authority estimated that 266 out of every 100,000 people have COVID-19, and with 20% of tests coming back positive, that number could be much higher. Still, some counties continue to see little to no confirmed transmission.

Antibody tests are very good at estimating the number of people who have been infected with SARS-CoV-2, the virus that causes COVID-19. But in a place like Oregon, where COVID-19 case rates are relatively low compared to other states, the tests aren’t very good at telling individual people if they’ve had the disease.

Through a quirk of statistics, depending on the accuracy of the test, it’s easy to get more false positives than true positives: which means a positive COVID-19 antibody test doesn’t necessarily mean you have antibodies for the virus.

Researchers can account for this mathematically, and use the rate of potential false positives and false negatives to estimate the general prevalence of the disease. Because research on COVID-19 immunity is still ongoing, it’s too soon to say if the people who have COVID-19 antibodies can get infected again.

Although the data is somewhat out of date and has a relatively small sample size, Cieslak says the results are important: they show that antibody tests can give a much better picture of the spread of COVID-19 than you get by simply counting all the cases. OHA hopes to continue to do further antibody surveys like this one, hopefully with more participants and stronger results.

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