At the tail end of the pandemic, and with around 80 percent of its employees vaccinated against COVID-19, Oregon’s flagship teaching hospital would seem like an unlikely place for a workplace-based coronavirus cluster.

And yet, there it was, buried in this week’s outbreak report from the Oregon Health Authority: 17 people listed as having tested positive for the virus. Those 17 people included staff members, medical students, patients, and visitors, all linked to the hospital’s adult cardiovascular care unit, known as 11-K.

Dr. Marcel Curlin, the medical director of Occupational Health at OHSU who was a part of the team investigating the outbreak, would not confirm how many of the 17 were staff members, nor how many had been fully vaccinated, but said all those who had been vaccinated and tested positive had asymptomatic or extremely mild cases of the virus.

About 80 percent of OHSU’s employees are vaccinated, most of them for months now; hospital employees were offered the Pfizer vaccine at on-campus clinics during the first waves of vaccinations back in December and January, Curlin says.

The incident, around which there are still some unanswered questions pending the completion of OHSU’s investigation, is notable as an outbreak in a hospital with strict protocols and a largely vaccinated medical population. As Curlin noted, however, the fact that those vaccinated who tested positive had such mild cases is proof that the vaccines are working as advertised, with extremely high effectiveness against serious complications, hospitalizations and death from COVID-19.  

Such so-called breakthrough cases are exceedingly rare, but not impossible, given that the currently available vaccines are upwards of 90 percent effective—which isn’t the same as 100 percent. But the protection against serious complications from COVID post-vaccination is even higher, which bore out in this case. Recently, the CDC announced that it would stop investigating such breakthrough cases among vaccinated people, unless they are hospitalized or die, in order to focus resources on understanding episodes of severe disease, though some researchers have said that is missing a chance to understand more about the long-term effects of COVID-19.

Curlin acknowledged that the situation is further complicated because staff members at OHSU are required to wear personal protective equipment—masks, and often protective eyewear—when in the hospital. Though the internal investigation is still underway, he says it’s possible that transmission can happen if protective equipment isn’t worn correctly 100 percent of the time, particularly considering that medical staff are in close proximity to their patients for extended periods of time.

“Somewhere along the line, that chain of protection was lost,” Curlin says.

Hospital investigators reviewed the possibility that transmission may have happened via ventilation or airflow, he says, but have been “reassured in that regard” as airflow in the rooms in question is at least four times higher than minimum standard. In this case, he says, transmission appeared to have happened the old-fashioned way—via droplets.

To be on the safe side, after the first infections were identified, PCR tests, which can pick up even trace amounts of the virus, were administered to all 111 employees on 11K, Curlin says. In a few cases, the virus was detected so minimally that it could not be cross-checked for genomic sequencing, but those cases were still counted as positives and affected employees asked to quarantine for 10 days for extra security, given the hospital setting.

Genomic sequencing of eight of the cases showed that they were linked to the outbreak, which began with a visitor to a patient on the floor; that visitor later tested positive for the virus. Three more cases have been confirmed as not linked; The hospital is still investigating the remaining 6 cases to pinpoint their origin.

As a result of the outbreak, OHSU may adjust when staff members are asked to wear N-95 masks. There are also ongoing conversations about developing “contracts” with COVID positive patients, Curlin says, including agreements on mobility, mask use and social distancing; patients who don’t adhere could be asked to leave the hospital.

“We don’t want people in solitary confinement and we need to make sure that we protect the people around them,” he says.