Lindsey Graham has become the first U.S. senator to disclose a breakthrough infection of COVID-19 after being vaccinated against the new coronavirus, saying Monday he is “very glad” he received the vaccine, without which his current symptoms would be “far worse.”

The news prompted several other lawmakers to get quick COVID-19 tests and report their status. A few of his senate colleagues spent part of the weekend working and socializing with the South Carolina Republican, who attended a gathering on Sen. Joe Manchin’s houseboat the evening that he first developed symptoms.

In a statement issued Monday afternoon, Graham said 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.”

According to the U.S. Centers for Disease Control and Prevention, fully vaccinated people determined to have been in close contact with someone who has COVID-19 do not need to quarantine if they are asymptomatic but should be tested within five days and wear a mask indoors for 14 days or until testing negative.

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 CDC 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.