COVID updates: Secretary of Defense Lloyd Austin tests positive

At least four U.S. cruise ships are reporting COVID-19 outbreaks aboard, leading to them being denied entry to -- or turned away from -- foreign ports.

Holland America's Line Koningsdam was denied entry into Puerto Vallarta on Thursday after 21 crew members tested positive.

Additionally, the Carnival Freedom ship was turned away from the Caribbean Islands of Bonaire and Aruba on Wednesday and Thursday, respectively, after "a small number" of passengers aboard tested positive for the virus.

Officials in Aruba and Curacao also turned away Royal Caribbean's Odyssey of the Seas on Wednesday after 55 fully vaccinated crew members and passengers tested positive.

On the same day, passengers aboard the Seven Seas Mariner were not allowed to disembark in Cartagena, Colombia, after six crew members and one passenger tested positive for COVID-19.

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."