Sen. Cory Booker D-NJ revealed on Twitter that he has tested positive with a breakthrough case of COVID-19.

Booker said he received both vaccine doses and his booster shot and his symptoms are "relatively mild."

He added that he's "certain" that if he had not received the shots, he "would be doing much worse."

Booker's announcement came hours after Sen. Elizabeth Warren D-Mass., revealed her breakthrough case. Warren also said her symptoms are mild.

In a tweet Sunday night, Sen. Elizabeth Warren, D-Mass., announced that she "tested positive with a breakthrough case" on Sunday. Warren said she's experiencing "mild symptoms."

The progressive senator also indicated that she is fully vaccinated and has received her booster shot.

New York state recorded over 20,000 positive COVID-19 cases for the third day in a row, Gov. Kathy Hochul's office said.

Sunday's total of 22,478 positive cases is the new single-day record, according to the data.

Over 542 patients were admitted to hospitals across the state for COVID-19 symptoms over the last 24 hours, the governor's office said.

There are 3,880 currently hospitalized in the state with the virus, the data showed.

Hochul reiterated her message to New Yorkers to get their vaccinations and boosters and follow health guidelines as the omicron variant continues to spread.

"This is not March of 2020, we are not defenseless," she said in a statement. "We have the tools to protect ourselves and the vulnerable loves ones in our families: Get vaccinated, get the booster and wear a mask when indoors or in large gatherings."

As of Sunday, 70.9% of all New York state residents have completed their vaccinations, the state data showed.

New York City Mayor Bill de Blasio warned that the city will have to brace for a jump in COVID-19 cases as the omicron variant continues to spread.

"We expect omicron to be a fast and temporary phenomenon. We expect these next weeks to see a very, very big surge in the number of cases, more than we've seen previously," he said during a news conference Sunday.

Roughly 162 patients were confirmed positive in the hospital, the mayor said. Hospitalizations have gone up, but not at the rate health officials were expecting.

"It will undoubtly grow a bit," de Blasio said.

The city recorded 5,731 new cases of COVID-19 Sunday.

The mayor urged New Yorkers to get vaccinated and to get their booster shots. The city's health department said as of Sunday 71% of all residents are fully vaccinated.

De Blasio also recommended that people wear masks indoors.

The city will focus on vaccination efforts for children and to increase booster access in nursing homes, he said.

At-home testing kits will be handed out at high volume testing sites, according to the mayor.

In Wisconsin, only 4% of ICU beds are available.

"This is getting really scary," Dr. Jamie Hess, an emergency physician at the University of Wisconsin, told ABC News.

"We're really reaching a crisis point where we have more patients to take care of then we have beds in the hospital or staff to care for them," Hess said.

Michigan has been struggling through a similar surge for nearly three months, with the state reporting more than 6,500 new cases each day. On average, more than 500 patients are being admitted to hospitals each day.

"Where we are right now feels a lot like the first surge back in March of 2020," Erin Dicks, a nurse manager at MICU Henry Ford Hospital, told ABC News. "We don't have enough beds to be able to manage this."

Dicks said so many patients are young.

"I think one of the biggest frustrating pieces for my staff is that they look at this as, this is preventable -- people don't have to die here," Dicks said.

-ABC News' Arielle Mitropoulos