Sens. Roger Wicker, R-Miss.; Angus King, I-Maine, and John Hickenlooper, D-Colo., said they tested positive after experiencing mild or limited symptoms and are currently isolating to stop further spread of coronavirus.
Positive COVID-19 tests among the vaccinated population are considered "breakthrough" cases amid the arrival of the highly contagious delta variant.
"I've tested positive for a breakthrough case of COVID-19," tweeted Hickenlooper. "I feel good but will isolate per docs instructions. I’m grateful for the vaccine (& the scientists behind it!) for limiting my symptoms."
FILE - Sens. John Hickenlooper, (D-CO), Roger Wicker (R-MS) and Angus King, I-Maine.
King still encouraged others to get vaccinated because he believes his symptoms would be much worse without it.
"While I am not feeling great, I’m definitely feeling much better than I would have without the vaccine," King said in a statement that described his symptoms as "mildly feverish."
"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," he added.
Wicker's communication director Phillip Waller said the senator sought a test due to "mild symptoms."
"Senator Wicker is fully vaccinated against COVID-19, is in good health, and is being treated by his Tupelo-based physician," Waller said in a statement. "He is isolating, and everyone with whom Senator Wicker has come in close contact recently has been notified."
Congress is not in session this week and lawmakers have been back in their home districts.
Since July, several other vaccinated members of Congress have announced positive coronavirus tests, including Sen. Lindsey Graham, R-S.C., and Reps. Sharice Davids, D-Kan., Vern Buchanan, R-Fla., Clay Higgins, R-La., and Ralph Norman, R-S.C.
Just this week, U.S. health officials announced plans to offer a third booster shot to Americans starting in September over concerns the Pfizer and Moderna vaccines' effectiveness is waning in the face of the delta variant.
Hickenlooper encouraged those who haven't got the shot to "get it today! And a booster when it’s available too."
About 72% of U.S. adults have received at least one coronavirus shot, according to the Centers for Disease Control and Prevention.