WASHINGTON, D.C. – U.S. Sen. Lindsey Graham says he has tested positive for the coronavirus.

In a tweet, the Republican lawmaker from South Carolina said Monday that he contracted the virus despite being vaccinated against COVID-19.

The 66-year-old man said he started to experience flu-like symptoms Saturday night and went to the doctor Monday morning.

“I feel like I have a sinus infection and at present time I have mild symptoms,” he wrote.

I was just informed by the House physician I have tested positive for #COVID19 even after being vaccinated.

I started having flu-like symptoms Saturday night and went to the doctor this morning.

— Lindsey Graham (@LindseyGrahamSC) August 2, 2021

Graham said he will be quarantining for 10 days.

He added that he’s fortunate to have received a COVID-19 vaccine because it will likely protect him from more serious illness.

“I am very glad I was vaccinated because without vaccination I am certain I would not feel as well as I do now,” Graham said. “My symptoms would be far worse.”

This is considered a “breakthrough case,” where a person vaccinated against COVID-19 still contracts the virus.

The Centers for Disease Control and Prevention (CDC) says these types of cases are expected because the vaccines are not 100% effective at preventing illness. However, the vaccines have been shown to be very effective at preventing serious illness and death in those who get them.