Washington: The coronavirus officially returned to the US Senate on Monday.

News that Sen. Lindsey Graham tested positive quickly jolted through the Capitol and sparked an hours-long scramble to figure out who else might have been exposed, after sources confirmed that the South Carolina Republican attended an outdoor event on Sen. Joe Manchin’s houseboat over the weekend with other senators.

Graham’s “breakthrough” case  the first known instance among senators, comes as the chamber has largely loosened social distancing restrictions in recent months, with at least 96 of the 100 senators vaccinated. Though the Capitol physician has recommended mask wearing regardless of vaccination status, the advice is not being strictly followed.

It also comes as the Senate is embarking on a tense slog of legislating: It is currently debating a bipartisan bill, which Graham has been helping advance, before Democrats turn to a budget resolution and then leave for a weeks-long break.