COVID updates: Secretary of Defense Lloyd Austin tests positive

Dr. Anthony Fauci said a COVID vaccine requirement for domestic air travel should be "seriously" considered.

"If you're talking about requiring vaccination to get on a plane domestically, that is just another one of the requirements that I think is reasonable to consider," he said Monday on MSNBC's "Morning Joe."

"When you make vaccination a requirement, that's another incentive to get more people vaccinated. If you want to do that with domestic flights, I think that's something that seriously should be considered," he said.

This is not the first time that Fauci has argued for vaccine mandates domestic flights.

On Sunday, Fauci told ABC's Jon Karl that "anything that could get people more vaccinated would be welcome."

-ABC News' Arielle Mitropoulos

Mayor Bill de Blasio said New York City has administered more than 180,000 COVID vaccine booster shots since a $100 incentive was announced on Tuesday.

This brings the total number of residents who have received boosters to more than 1.9 million.

"We did a booster incentive and it has been heard and felt by the people of NYC," de Blasio told MSNBC on Monday.

-ABC News' Aaron Katersky

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."