Four people in Oregon have tested positive for the coronavirus after receiving both doses of the Covid-19 vaccine, health officials said.
There are two cases each in Yamhill and Lane counties, the state's Health Authority said in a series of tweets on Friday. The cases are either mild or asymptomatic.
"We are working with our local and federal public health partners to investigate and determine case origin," the agency said. "Genome sequencing is underway, and we expect results next week."
The agency referred to the individuals who tested positive as "breakthrough cases," meaning that they got sick with the virus at least 14 days after receiving both doses.
The Health Authority said more breakthrough cases could pop up.
"Clinical trials of both vaccines presently in use included breakthrough cases. In those cases, even though the participants got Covid, the vaccines reduced the severity of illness," the agency said in a tweet.
"Based on what we know about vaccines for other diseases and early data from clinical trials, experts believe that getting a Covid-19 vaccine may also help keep you from getting seriously ill even if you do get the virus. ... Getting as many Oregonians as possible vaccinated remains a critical objective to ending the pandemic."
The agency's announcement came the same day its health officer said there has been a decline of daily Covid-19 cases over the past several weeks. As of Friday, there were 149,576 cases in the state, according to the department's count.
"These decreases are a testament to the actions all Oregonians are taking to slow the spread of Covid-19 and the sacrifices made – thank you," health officer Dean Sidelinger said at a news conference Friday.
Another breakthrough case was reported in North Carolina, according to NBC affiliate WCNC-TV in Charlotte. The state's Department of Health and Human Services told the outlet that the person had mild symptoms and did not need to be hospitalized.
The Centers for Disease Control and Prevention has said that quarantining is not necessary for fully vaccinated people within three months of having received their last doses as long as they do not develop any symptoms.
They do, however, still need to practice certain safety measures such as wearing face masks, social distancing, and avoiding crowds or poorly ventilated spaces.
"Fully vaccinated" means at least two weeks have passed since a person has completed their vaccination series.