Sen. Lindsey Graham may have exposed multiple senators to COVID-19 on Joe Manchin's DC houseboat

• Sen. Joe Manchin hosted multiple senators, including Lindsey Graham, on his houseboat over the weekend.
• Graham announced that he tested positive for COVID-19 on Monday morning.
• Manchin and Sens. Jacky Rosen and Mark Kelly were all in contact with Graham on the boat.

Sen. Joe Manchin, a West Virginia Democrat, hosted a group of senators on his Washington, DC houseboat over the weekend. Guests included Republican Sen. Lindsey Graham, who announced that he tested positive for COVID-19 on Monday, despite being fully vaccinated.

Manchin, who's also vaccinated, said he tested negative for COVID-19 on Monday. The other senators in attendance were Democrats Mark Kelly of Arizona, Chris Coons of Delaware, Jacky Rosen of Nevada, Catherine Cortez Masto of Nevada, Maria Cantwell of Washington, and Republican John Thune of South Dakota. Manchin said the bipartisan gathering was "all outdoors" and that everyone was vaccinated.

Spokespeople for Thune, Rosen, and Coons said the lawmakers tested negative for COVID-19 on Monday. The other senators' offices didn't immediately respond to Insider's request for comment.

Manchin insisted the event wasn't a "party" and Coons suggested the conditions of the gathering weren't ripe for spreading COVID-19.

"It was a cookout. I mean - we literally had hot dogs 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.

A spokesperson for Manchin declined to provide Insider with the list of people who gathered on Manchin's houseboat, "Almost Heaven," which is where the veteran lawmaker lives while he's in DC.

The CDC recommends that fully-vaccinated people exposed to someone infected with COVID-19 get tested 3-5 days later and wear a mask indoors for two weeks or until they test negative for the virus.

Graham announced via tweet that he tested positive on Monday morning after coming down with "flu-like symptoms" on Saturday. Graham received the Pfizer vaccine last December and expressed relief at having been immunized prior to his infection.

"I feel like I have a sinus infection and at present time I have mild symptoms," he wrote on Twitter. "I will be quarantining for ten 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."

COVID-19 cases are surging across the US, particularly in areas where vaccination rates are low. The US's weekly average of new daily cases has more than quintupled: growing from just over 12,000 at the end of June to nearly 72,000 last Thursday.

New data shows that vaccinated people can still experience so-called breakthrough infections, although these cases are generally mild, according to the CDC. Delta is the most transmissible strain of the virus and it now makes up more than 80% of US infections.

While the House has a mask mandate in place, the Senate no longer requires masks. Almost all senators have said they're vaccinated, while dozens of House members won't say whether they're vaccinated. Senate Democrats briefly moved back to in-person meetings but have resumed holding their leadership meetings virtually amid the surge in cases.