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.

Starting Dec. 22, the U.S. could be on Israel's no-fly list due to high COVID infection rates. All travel from Israel to the U.S. will be banned, the Israeli government announced Sunday.

The Ministry of Health has recommended that the U.S., and nine other countries including Canada, be classified as "red list" countries. The decision now awaits final approval from the Knesset's Constitution, Law and Justice Committee.

Israeli citizens coming to Israel from the U.S. will have to go into full isolation, including those vaccinated or recovered. Non-Israeli citizens coming from the U.S. will not be allowed to enter Israel, officials said.

At the start of the weekly cabinet, Israel Prime Minister Naftali Bennett said the fact the country is in a "relatively good situation" is "not a coincidence." He attributed delaying the entry of the variant to Israel to travel bans. "What a pity that other countries did not do as we did," he said Sunday.

In addition to the U.S., Israel also will also move Belgium, Canada, Germany, Hungary, Italy, Morocco, Portugal, Switzerland and Turkey to the no-fly list this week on Dec. 22. Norway, France, Spain, Sweden and the UAE will be added to Israel's no-fly list at midnight Dec. 20.

There are 52 countries on Israel's red list.

-ABC News' Christine Theodorou

Pfizer's forthcoming COVID-19 treatment could be authorized for emergency use in the United States by the end of this year or early next year, pending the Food and Drug Administration's regulatory timeline, Pfizer CEO Albert Bourla told ABC News.

The pharmaceutical giant announced Tuesday that it has submitted promising new data to the FDA, including lab experiments showing its oral antiviral medicine, Paxlovid, will likely work against the omicron variant. And in updated clinical trial data, Pfizer found the treatment reduces the risk of being hospitalized or dying by 89% when taken within three days of being diagnosed with COVID-19, and 88% when taken within five days of being diagnosed among high-risk, unvaccinated patients.

"It is a game changer," Bourla told ABC News. "But at the same time, I want to emphasize that no one should use the existence of the pill as an excuse to avoid vaccination."

Bourla said the medicine could save thousands of lives.

"We did some calculations how many deaths or hospitalizations can be avoided based on the current U.S. mortality rates and the current U.S. hospitalization rates," he said. "On the back-of-the-envelope calculation, we estimate that 100,000 people if they take the pill, we will avoid 6,000 hospitalizations and 1,200 deaths approximately."

-ABC News' Sony Salzman