Senator Lindsey Graham announced Monday that he tested positive for Covid-19, but the Republican from South Carolina said his “symptoms would be much worse” if he hadn’t been given the full dose of the coronavirus vaccine.

Graham, 66, said in a tweet that he had flu-like symptoms on Saturday night and that he would be quarantined for 10 days.

While the Senate has not officially issued a mask mandate, Graham was seen wearing a mask on Monday, according to a Politico reporter.

Graham began to show symptoms when the Senate met over the weekend to work on the bipartisan infrastructure bill. It is unclear how many other members of the Senate came into close contact with him.

Graham attended a Saturday night party aboard Senator Joe Manchin’s, DW.V., houseboat, NBC News confirmed. Other Senators in attendance were Mark Kelly, D-Ariz., Chris Coons, D-Del., John Thune, RS.D. and Jacky Rosen, D-Nev., whose offices and NBC have been certified.

Manchin and Kelly spokesmen said the senators are fully vaccinated and follow CDC guidelines for anyone exposed to a person who tests positive. Thune’s office and Rosen confirmed that the senators tested negative for Covid-19 on Monday.

CDC guidelines include testing for Covid three to five days after exposure to someone with a confirmed or suspected case and wearing a mask indoors in public for 14 days or until the test is negative, the CDC said. Fully vaccinated people should also isolate themselves if they test positive.

According to the guidelines, senators appear to be able to vote in masked form unless they get a positive test result.

Graham was one of 17 Republican senators who last week voted to move the infrastructure plan forward. His absence or possible quarantine for other senators could affect the passage of the bill as a final vote approaches.

While the Senate has not reintroduced a mask mandate, the Republicans in the House of Representatives have resolutely opposed a renewed mask requirement in their chamber. The House of Representatives has more than four times as many members as the Senate and a larger proportion of unvaccinated MPs.

Graham’s announcement came as the United States is grappling with a spike in the Delta variant of Covid and the Biden government is urging more people to get vaccinated.

The CDC warned in a confidential document Thursday that the Delta variant, which is the dominant form of the disease in the United States today, 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 serious illnesses but may be less effective at preventing infections.

The confidential document, which has been reviewed by CNBC, states that there are 35,000 symptomatic infections in 162 million vaccinated Americans every week.

Publicly available CDC data says that by July 19, 5,914 fully vaccinated people with Covid infections had been hospitalized or had died from Covid infections – the vaccinated are generally milder.

Data from a survey by Monmouth University shows a partisan split in reluctance to take Covid vaccines.

The survey found that 17% of respondents said they were likely to refuse the vaccine. Among them, 70% either identify with or tend to identify with the Republican Party, while only 6% identify with the Democrats.

The Biden administration continues to urge more Americans to get vaccinated, especially as the US prepares for an expected surge in Delta Covid infections in the fall.

On Monday, the U.S. hit Biden’s May target of providing 70% of U.S. adults with at least one vaccination, about a month behind the original July target.

CDC says the US reports an average of about 600,000 vaccinations a day through Sunday. This is far from the more than 3 million daily recordings in mid-April, but an increase of 14% over the previous week.

Last week, Biden announced that federal employees would need to prove their Covid vaccination status or undergo safety protocols.

Biden had previously advised federal agencies not to require vaccination for field work, but the Department of Veterans Affairs was the first federal agency to instruct health workers to receive the Covid vaccine.

Biden, who is fully vaccinated, also said he will follow the reverse CDC guidelines for fully vaccinated individuals to wear masks indoors in areas with high transmission rates.

– CNBC’s Berkeley Lovelace Jr. and Dan Mangan contributed to this report

This story evolves. Please check again for updates.