The surge in COVID-19 cases is "going to get worse before it gets better, that's for sure," Dr. Anthony Fauci said Monday on "Good Morning America."

And it could take weeks for the number of daily cases to start declining, he noted.

"It's very difficult to predict, because we have so many unvaccinated people in this country who are really quite vulnerable," Fauci said. "Even with the vaccinated people, you're going to see breakthrough infections."

Fauci also sought to ease concerns about the availability of at-home tests. Demand for those tests spiked during the holiday season, as worries about omicron grew and families took precautions before gathering.

"Well certainly omicron is a very, very unusual variant in its incredible degree of spreading," Fauci said. "As we enter January, there will be many, many more tests that we have."

Medical staffers at Tel HaShomer hospital near Tel Aviv, Israel, have begun receiving a fourth COVID-19 vaccine dose.

About 150 staffers were taking part in the trial, which began on Monday morning. They'll be monitored for a week.

The Ministry of Health has yet to announce a final decision on a fourth jab. Prime Minister Naftali Bennett's office last week said on Twitter that it had instructed the government to "prepare for an extensive operation."

"This is wonderful news that will assist us in getting through the Omicron wave that is engulfing the world," Bennett said in a statement.

-ABC News' Bruno Nota

Subway trains in New York City will run less frequently than usual this week amid a spike in COVID-19 cases.

"Like everyone in New York, we've been affected by the COVID surge," New York City Transit Authority officials wrote on Twitter.

Officials said the subway was dealing with staff shortages and service would be scaled back until Thursday.

"We’re taking proactive steps to provide the best, most consistent service we can," officials said on Twitter. "That means you may wait a little longer for your train."

A man in his 80s who died near Sydney, Australia, was the country's first known death linked to the omicron variant, health officials said on Monday.

"The man was a resident of the Uniting Lilian Wells aged care facility in North Parramatta, where he acquired his infection," New South Wales health officials said in a news release. "He had received two doses of a COVID-19 vaccine and had underlying health conditions."

The country’s new daily cases topped 10,000 on Sunday, local media reported on Monday.

In New South Wales, Australia's most populous state, new coronavirus infections dipped to 6,324 on Sunday, down from a record 6,394 new cases on Christmas Day, according to health officials.

Rep. Ayanna Pressley, D-Mass., is the latest lawmaker to test positive for COVID-19. Pressley announced the news Friday afternoon, saying she was experiencing "relatively mild " symptoms and isolating.

She also took the opportunity to say she was vaccinated and boosted and advocate for others to do the same.

"Vaccines save lives," she said in a statement. "With this unprecedented pandemic continuing to rage, I am deeply grateful for the scientists, researchers, and frontline healthcare workers who have worked tirelessly to develop vaccines that are safe and effective, and ensure that our communities are protected."

Pressley, who has garnered national attention as a member of "The Squad" alongside Reps. Alexandria Ocasio-Cortez, Rashida Tlaib and Ilhan Omar, is one of more than a dozen members of Congress to test positive in the past two weeks. Illinois Rep. Chuy Garcia announced Thursday that he had also tested positive for a breakthrough case and was experiencing mild symptoms.