U.S. Sen. Lindsey Graham has tested positive for the coronavirus, the first senator to disclose a breakthrough infection after being vaccinated.

In a statement issued Monday afternoon, the South Carolina Republican said that he “started having flu-like symptoms Saturday night” and went to the doctor Monday morning.

After being notified of his positive test, Graham said he would quarantine for 10 days.

“I feel like I have a sinus infection, and at present time, I have mild symptoms,” the 66-year-old Graham said. “I am very glad I was vaccinated because without vaccination, I am certain I would not feel as well as I do now. My symptoms would be far worse.”

Graham, who was vaccinated in December, has long been a proponent of vaccination, saying during a visit this spring to the Medical University of South Carolina in Charleston that “the sooner we get everybody vaccinated, the quicker we can get back to normal.”

Graham’s infection comes on the heels of updated Centers for Disease Control and Prevention guidance urging even fully vaccinated people to return to wearing masks indoors in areas of high coronavirus transmission, citing the surge of the highly contagious delta variant. Recent analysis has shown that breakthrough cases of COVID-19, with mild or no symptoms, still remain rare.

Both congressional chambers have been adopting stricter face covering regulations amid a resurgence in COVID-19 cases across the country. The House has returned to a mask requirement, while the Senate has recommended medical-grade face coverings.

Graham, who was wearing a mask, did not answer questions from reporters on Capitol Hill earlier Monday, prior to the statement being issued. Because of Senate votes, Graham was not in attendance at Friday night’s Silver Elephant dinner, the South Carolina Republican Party’s signature annual fundraiser and an event attended by hundreds, including former Secretary of State Mike Pompeo, a headlining speaker.