In this May 28, 2021, file photo, U.S. Sens. Angus King, I-Maine, left, and Joe Manchin, D-West Virginia, walk to the chamber at the Capitol in Washington. Credit: J. Scott Applewhite / AP

U.S. Sen. Angus King tested positive for COVID-19, his office announced Thursday.

King, a 77-year-old independent who caucuses with Democrats, said he took a test Thursday morning on a doctor’s recommendation a day after feeling mildly feverish.

“While I am not feeling great, I’m definitely feeling much better than I would have without the vaccine. I am taking this diagnosis very seriously, quarantining myself at home and telling the few people I’ve been in contact with to get tested in order to limit any further spread,” he said in a statement.

King is the highest-profile Maine politician to report testing positive for COVID-19 since the pandemic began. Several other senators have tested positive in recent weeks despite being fully vaccinated. Sens. Roger Wicker, R-Mississippi, and Lindsey Graham, R-South Carolina, also contracted the virus earlier this month. Both likewise said they had mild symptoms.

These breakthrough cases of COVID-19 remain uncommon, although health officials have indicated they are likely to authorize booster shots next month due to a slight decline in vaccine effectiveness in preventing infections. Severe illness, hospitalization and death among vaccinated people remain even rarer.

Prior to getting vaccinated, King would have been considered to be at a higher risk for severe COVID-19 due to his age. He has had multiple bouts with cancer, first as a young Senate staffer in the 1970s. He had successful prostate cancer surgery in 2015 and most recently underwent radiation treatment for what he called “residual” prostate cancer in 2019.

The former two-term governor said Thursday that he encouraged Mainers who are not yet vaccinated to get the vaccine. Cancer patients and survivors have been shown to be more vulnerable to the virus, but vaccines all but eliminate the risk of serious illness and death.