The South Carolina Republican said in a tweet that he began having flu-like symptoms Saturday night and went to the U.S. House physician Monday morning.
Graham attended a Saturday night party on fellow Senator Joe Manchin’s DC-based “Almost Heaven” house boat, according to Graham’s office, NBC News reported.
“I feel like I have a sinus infection and at present time I have mild symptoms,” said Graham, adding that he will quarantine for 10 days.
“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.”
Other senators present at the party included Mark Kelly, D-Ariz., Chris Coons, D-Del., John Thune, R-S.D., and Jacky Rosen, D-Nev., their offices and NBC News confirmed. Catherine Cortez Masto, D-Nev., and Maria Cantwell, D-Wash., also attended, their offices confirmed with NBC News.
Speaking to reporters on his way out of the Capitol Monday, Manchin said he did not have a party, instead saying, “When you say party, there’s no parties, basically there’s gatherings we have on ‘Almost Heaven’… so we know each other and talk to each other.”
“We were outside. Okay? And we were all, everybody’s been vaccinated,” Manchin said when reporters asked him about the party, adding that Graham was “all good.”
When asked how long the party went on for, Manchin said, “Oh I don’t know whatever it takes [to] eat a hamburger or two … all outdoors.”
Spokespeople for Manchin and Kelly said the senators are fully vaccinated and following CDC guidelines for people exposed to a person who tested positive.
Thune’s office and Rosen confirmed the senators tested negative for Covid-19 on Monday. Coons told NBC that he tested negative, and Cantwell’s office confirmed that she is getting tested.
“It was a cookout. I mean — we literally had hotdogs and hamburgers at like four in the afternoon on the top of a boat that was moving in a, you know, stiff breeze,” Coons told NBC News.
The CDC guidelines include testing for Covid three-to-five days after exposure to someone with a confirmed or suspected case, and wearing a mask in public indoor settings for 14 days or until they test negative, according to the CDC. Fully vaccinated individuals should also isolate if they test positive.
Under the guidelines, senators would appear to be able to attend votes while masked unless they receive a positive test result.
The lead Republican infrastructure negotiator, Sen. Rob Portman of Ohio, was invited to Manchin’s boat but did not attend the gathering, he told CNBC on Tuesday. He got a Covid-19 test after spending time around Graham in the Senate and it came back negative, he said.
Graham was one of 17 Republican senators who voted to advance the infrastructure plan last week. His absence, or a potential quarantine for other senators, could affect passage of the bill if a final vote is closer.
While the Senate has not reinstated a mask mandate, House Republicans have strongly opposed a renewed mask requirement in their chamber. The House has more than four times as many members as the Senate and a larger share of unvaccinated lawmakers.
Graham’s announcement came as the United States contends with a surge of the delta variant of Covid, and as the Biden administration urges more people to get vaccinated.
The CDC on Thursday warned in a confidential document that the Delta variant, which is now the dominant form of the disease in the U.S., is as contagious as chickenpox and has a longer transmission window than the original Covid strain.
Health officials said Covid vaccines prevent more than 90% of severe disease, but may be less effective at preventing infection.
The confidential document, which was reviewed by CNBC, said that 35,000 symptomatic infections are occurring each week among 162 million vaccinated Americans.
Publicly available CDC data states that 5,914 fully vaccinated individuals had been hospitalized or died with Covid infections as of July 19. But health officials have also pointed out that the majority of people hospitalized or killed by Covid are unvaccinated, and that “breakthrough infections” among the vaccinated tend to be milder.
Data from a Monmouth University poll show a partisan split in Covid vaccine hesitancy.
The survey found 17% of respondents said they were likely opposed to getting the vaccine. Among them, 70% either identify or lean toward the Republican Party, while merely 6% identify with Democrats.
The Biden administration is continuing to push for more Americans to get vaccinated, especially as the U.S. prepares for an expected surge in Delta Covid infections in the fall.
On Monday, the U.S. reached Biden’s goal set in May to have 70% of U.S. adults with at least one shot of the vaccine, about one month behind the original July goal.
CDC indicates that the U.S. is reporting an average of approximately 600,000 vaccinations per day as of Sunday. This is far from the more than 3 million daily shots in mid-April, but up 14% from the week prior.
Last week, Biden announced that federal workers will be required to prove their Covid vaccination status or submit to safety protocols.
Biden had previously discouraged federal agencies from requiring vaccination for on-site work, but the Department of Veterans Affairs became the first federal agency to order health-care workers to get the Covid vaccine.
Biden, who is fully vaccinated, also said he would follow the CDC’s reversed guidance for fully-vaccinated people to wear masks indoors in areas where transmission rates are high.
— CNBC’s Berkeley Lovelace Jr. and Dan Mangan contributed to this report
This story is developing. Please check back for updates.
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