In this June 18, 2021, file photo, U.S. Sen. Angus King, I-Maine, speaks at Acadia National Park in Winter Harbor, Maine. Credit: Robert F. Bukaty / AP

U.S. Sen. Angus King tested positive for COVID-19 on Thursday, making for a rare case in a fully vaccinated Mainer, and emphasizing the continued importance of shots in preventing severe illness in people who would otherwise be vulnerable.

The Maine independent was one of three senators to announce a positive COVID-19 test on Thursday alone as the highly contagious delta variant pushes case numbers up nationwide. Sens. John Hickenlooper, D-Colorado, and Roger Wicker, R-Mississippi, also said they were experiencing mild symptoms.

It is not clear where King, who said he has followed public health precautions since the start of the pandemic, contracted the virus. His office said he has been in Maine for the past week since the Senate finished a long series of votes, but he has had some meet-ups outdoors.

King, who is 77, would fall into a group of older people vulnerable to serious illness from the virus if he had not been fully vaccinated in January. He has also had three bouts with cancer since the 1970s, most recently being treated for prostate cancer in 2019. But he said in a Thursday statement that he is only experiencing minor symptoms and quarantining at home.

Breakthrough COVID-19 cases, like those experienced by King and the other senators, were always expected. Even in clinical trials, the Pfizer and Moderna vaccines were only about 95 percent effective in preventing symptomatic illness, though they perform even better in terms of preventing severe illness and death.

The Maine Center for Disease Control and Prevention has documented only 863 cases in more than 860,000 fully vaccinated people here, although the agency acknowledges that figure is likely an undercount as not all cases are investigated to determine vaccine status. Serious cases are far rarer. There have been 35 hospitalizations and 14 deaths among vaccinated people, according to state data.

Breakthrough cases may be more common due to the highly contagious delta variant, said Dr. Peter Millard, a physician and epidemiologist in Waldo County. But while age, a compromised immune system or other comorbidities may make someone more susceptible to the virus, being vaccinated still “probably protects them from getting very ill,” Millard said.

He said the prevalence of breakthrough cases would likely lead to the use of booster shots in the next few months, pointing to recent data out of Israel, which has begun to administer third doses to people aged 60 and older after seeing rising cases among its vaccinated population.

The U.S. appears poised to begin offering third doses in September. Citing data showing the vaccines have become marginally less effective in preventing symptomatic COVID-19, U.S. Centers for Disease Control and Prevention Director Rochelle Walensky said booster shots will likely be recommended next month for individuals who received the vaccine eight months prior.

Despite increasing breakthrough cases, the vast majority of COVID-19 infections in Maine continue to be among unvaccinated people, state data suggest. The vaccine still makes people significantly less likely to contract the virus, Millard noted, and less likely to get severely ill if they do. King likewise encouraged Mainers who are not yet vaccinated to do so.

“Remain vigilant, follow the guidance from health professionals, and get vaccinated if you haven’t been,” he said.