Sen. Elizabeth Warren, D-Mass., and Sen. Cory Booker, D-N.J., said Sunday that they have tested positive for the coronavirus, becoming the latest fully vaccinated members of Congress to experience breakthrough infections.

“I regularly test for COVID & while I tested negative earlier this week, today I tested positive with a breakthrough case,” Warren wrote on Twitter.

Warren, who had received three shots of a coronavirus vaccine, added that she was experiencing only mild symptoms and was “grateful for the protection provided against serious illness that comes from being vaccinated & boosted.”

Booker said Sunday that he first felt symptoms the previous day and that they were “relatively mild.”

“I’m beyond grateful to have received two doses of vaccine and, more recently a booster — I’m certain that without them I would be doing much worse,” Booker said in a statement.

Though it is unclear which coronavirus variant infected Warren and Booker, their positive tests come as top government health officials are warning that the United States will probably see record numbers of coronavirus cases and hospitalizations with the rapid spread of the new omicron variant.

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On CBS News’ “Face the Nation” on Sunday, Francis Collins, director of the National Institutes of Health, said cases will increase steeply over the next couple of weeks and that the country could soon see 1 million new cases a day of omicron, dramatically exceeding the record of about 250,000 new cases per day set in January.

Dr. Anthony Fauci, the nation’s leading infectious-disease expert, described omicron as “extraordinary” in its transmissibility, with a doubling time of two to three days.

“It is going to be a tough few weeks, months as we get deeper into the winter,” Fauci said.

In the early days of the pandemic, before vaccines were available, Warren lost her eldest brother to COVID-19. Donald Reed Herring died in Oklahoma in April 2020, just a few weeks after testing positive for the virus at age 86.

“He was charming and funny, a natural leader,” Warren wrote on Twitter then. “What made him extra special was his smile — quick and crooked, it always seemed to generate its own light, one that lit up everyone around him.”