Three senators announced within hours of each other on Thursday that they had tested positive for the coronavirus, despite each being fully vaccinated.

Sens. Roger Wicker (R-Miss.), Angus King (I-Maine) and John Hickenlooper (D-Colo.) said they had tested positive in what is known as a breakthrough case, when fully vaccinated individuals test positive for COVID-19. It marks three known breakthrough cases among senators within 24 hours.

The positive tests come as senators have been back in their home states for roughly a week and aren't expected to return to Washington, D.C., until mid-September.

Hickenlooper became the latest on Thursday afternoon to announce he had tested positive, saying he tested positive after "experiencing mild symptoms." 

"I’m feeling much better and will continue to isolate at the direction of the Congressional Attending Physician," he said

Though Hickenlooper announced his positive test Thursday, his statement didn't specify when he tested positive. Spokespeople for the senator didn't immediately respond to a question about when he got tested. 

Both Wicker and King — the Senate's second and third breakthrough cases, respectively, after Sen. Lindsey Graham's (R-S.C.) positive test earlier this month — said they tested positive after experiencing symptoms. 

King, in his statement, said that while he and his staff took precautions to social distance while in D.C., that he began "feeling mildly feverish" on Wednesday and got tested in Maine on Thursday.

"It came back positive. While I am not feeling great, I’m definitely feeling much better than I would have without the vaccine. I am taking this diagnosis very seriously, quarantining myself at home and telling the few people I’ve been in contact with to get tested in order to limit any further spread," King said in a statement.

Phillip Waller, a spokesperson for Wicker, added that the Mississippi Republican tested positive on Thursday, and that he got tested because he was experiencing "mild symptoms."

"Senator Wicker is fully vaccinated against COVID-19, is in good health, and is being treated by his Tupelo-based physician. He is isolating, and everyone with whom Senator Wicker has come in close contact recently has been notified," he said.

Symptoms for COVID-19 can appear between two to 14 days after exposure to the virus, according to the Centers for Disease Control and Prevention (CDC).

Senators spent hours on the Senate floor together last week, on Aug. 11, before leaving town. 

The Senate was in session for roughly 20 hours starting Tuesday morning through early Wednesday to pass a bipartisan infrastructure bill and approve Democrats' budget, with senators together for much of that time. 

Their announcements Thursday bring the total number of known COVID-19 breakthrough cases among senators to four. Graham became the first earlier this month when he announced that he had tested positive despite being fully vaccinated.

Graham quarantined and returned to the Capitol last week in time to vote against Democrats' $3.5 trillion budget resolution.

"Sending best wishes for a speedy recovery to my good friends and colleagues," Graham tweeted on Thursday while urging his followers to get vaccinated.

The daily average number of cases for the United States has climbed upward in recent weeks, but still is significantly lower than the peak reached late last year and in early January. 

Public health officials have stressed that symptomatic breakthrough infections for fully vaccinated individuals are rare and getting vaccinated makes getting a severe case of the coronavirus less likely.

A CNN survey earlier this year found that at least 96 of 100 senators had been vaccinated. Testing is also available within the Capitol complex. 

And according to data from the CDC, less than 1 percent of fully vaccinated Americans have had a breakthrough case that results in hospitalization or death.

Both King and Hickenlooper urged unvaccinated individuals to get vaccinated, crediting it with keeping them from developing more severe symptoms. 

"I’m grateful for the vaccine (and the scientists behind it) for limiting my symptoms and allowing us to continue our work for Colorado. If you haven’t been vaccinated, don’t wait for the virus — get the shot today, and a booster when it’s available too!" Hickenlooper said.

King also urged "everyone to remain vigilant, follow the guidance from health professionals, and get vaccinated if you haven’t been."