The study, conducted by the Oregon Health Authority, found nine of 897 people tested from May 11 to June 15 had coronavirus antibodies – indicating a previously undetected infection.

Oregon’s findings underscore what public health officials have long said: Only a fraction of active infections are being identified through diagnostic testing. But it’s difficult to draw sweeping conclusions from the relatively small number of antibody cases identified in the study.

“The data suggest that a substantial number of COVID-19 cases in Oregon have gone undiagnosed and not reported and that a large portion of Oregon’s population remains susceptible to COVID-19 infection,” state officials wrote in the study, published as part of a weekly report by the Centers for Disease Control and Prevention.

Unlike diagnostic tests that identify active infections, serology tests analyze blood to determine if someone has previously been infected by identifying the presence of antibodies. It can often take two to three weeks after an infection for someone to develop those antibodies.

Dr. Paul Cieslak, one of the authors, said the results were “pretty close to what we expected.” He cautioned that with only nine identified antibody cases in the study, “there’s a margin of error around that that is going to be considerable,” with a range of .2% to 1.8%.

“We don’t want to make more of this than it was,” he said. “But it did sort of confirm what we thought, that there’s a bunch of undiagnosed infections out there.”

Oregon’s study provides some support for modeling that officials have used for months to analyze spread during the pandemic. The modeling last month, for example, estimated 52,400 Oregonians had been infected as of July 2 – even though only one-fifth of those had been identified through diagnostic testing.

Findings from the antibody study, if extrapolated to the entire state’s population, would equal roughly 42,000 infections as of June 15.

“I don’t think that our results are terribly inconsistent with the modeling, let me say that,” Cieslak said.

Oregon’s latest modeling estimated 88,800 cumulative infections through July 30, again with only about one in five identified through diagnostic testing.

To complete the antibody study, officials recruited 19 facilities across Oregon to provide patients’ blood samples to the state public health laboratory for testing. They found that older Oregonians were more likely to have antibodies among those screened.

The testing did not identify antibodies in 29 children tested and found only two people with antibodies among 485 adults 64 and younger.

But among Oregonians 65 and older, seven people out of 383 tested had antibodies.

The state launched its study in May one week after telling The Oregonian/OregonLive it wasn’t analyzing the antibody test results it received from labs to estimate the prevalence of infections. With so many questionable antibody tests on the market, state officials said they didn’t trust the results.

But Cieslak said the state tempered those concerns by conducting the lab work for the study in-house using a serology test it selected.

State officials have said more antibody studies are planned, including in long-term care facilities and schools. Cieslak said he’s not sure when that might happen.

The state’s findings generally match estimates from federal officials that only about one in 10 infections has been identified through diagnostic testing since the start of the pandemic. Officials in Boise, meanwhile, completed a similar antibody study that found previous infections in 1.79% of people tested.

As of Friday, the state has reported 22,613 confirmed or presumed infections from diagnostic testing. Aside from the new study, Oregon does not report antibody test results.

“We’re in this for the duration,” Cieslak said, noting that millions of Oregonians have not been infected with coronavirus. “We need to keep the social distancing until we get very good treatments or a reliable vaccine.”

This article was originally published by The Oregonian/OregonLive, one of more than a dozen news organizations throughout the state sharing their coverage of the novel coronavirus outbreak to help inform Oregonians about this evolving heath issue.