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.

Last week, the CDC reported that omicron had overtaken delta to become the dominant COVID-19 variant in the U.S. However, on Tuesday, the CDC revised its initial estimates to noticeably lower numbers of omicron cases.

Last Monday, the CDC said omicron was estimated to makeup more than 73.2% of new cases as of Dec. 18. Now, the CDC says just 22.5% of new U.S. cases were estimated to be omicron as of Dec. 18. 

According to new data from this week, the CDC says omicron is now estimated to account for 58.6% of all new cases, officially making it the dominant strain in the U.S. In the New York and New Jersey region as well as the deep South, omicron is estimated to account for more than 85% of new cases. 

A CDC spokeswoman said in a statement, "There was a wide predictive interval posted in last week’s chart, in part because of the speed at which omicron was increasing. We had more data come in from that timeframe and there was a reduced proportion of omicron. It’s important to note that we’re still seeing steady increase in the proportion of omicron."

ABC contributor Dr. John Brownstein, an epidemiologist at Boston Children's Hospital, said, "It may be possible that a large portion of their [the CDC's] sequencing data had come from labs that were specifically looking for s-gene samples, thus falsely elevating the percent omicron in samples."

-ABC News' Arielle Mitropoulos