The Centers for Disease Control and Prevention (CDC) warned Americans to avoid cruises regardless of COVID vaccination status.

On Thursday, the CDC raised the health notice from a level 3 to a level 4, which is the highest level.

"Since the identification of the omicron variant, there has been an increase in the number of COVID-19 cases among cruise passengers and crew reported to CDC," the agency wrote. "Additionally, there has been an increase in the number of cruise ships meeting the COVID-19 case threshold for CDC investigation."

More than 80 cruise ships are currently being investigated or observed by the CDC after reports of COVID outbreaks on board.

In response, the Cruise Lines International Association -- the largest cruise trade association -- called the CDC's new warning "particularly perplexing" and said cases on ships "consistently make up a very slim minority of the total population onboard."

ABC News' Alexandra Faul, Mina Kaji and Sam Sweeney

COVID-19 vaccines are safe and effective in children, three new studies published Thursday by the Centers for Disease Control and Prevention find.

The first study, looking at the Pfizer vaccine in 42,000 kids aged 5 to 11 found that side effects from the shot were mostly mild and resolved within a week.

The second study, which examined data from 243 adolescents between ages 12 and 17, found the Pfizer vaccine was 92% effective at preventing COVID infection.

The final study found less than 1% of eligible children between ages 5 and 17 who were hospitalized with COVID in July and August were fully vaccinated.

"I think these studies taken as a whole confirm what we’ve been saying all along, which is that these vaccines are incredibly safe and effective," ABC News medical contributor Dr. John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital, said.

ABC News' Katie Kindelan

About 75% of people in the U.K. experiencing new cold-like symptoms actually have symptomatic COVID-19, a new study suggests.

Health science company ZOE, which has been conducting the ongoing study with King's College London, said the figure is based on a decline in the number of non-COVID "colds" and a sustained increase in symptomatic COVID cases.

Dr Claire Steves, scientist on the ZOE COVID Study app and Reader at King’s College London, said she wants the U.K. government's list of symptoms to be updated to include headache, runny nose and sore throat.

"The fact that 75% of new cold-like symptoms are COVID, and the classic symptoms are much less common, means the government advice needs to be urgently updated," she said in a release Thursday.

On Wednesday, the U.K. recorded a record-high 183,037 cases of COVID-19, more than double the figure reported at this time last year.

More free COVID-19 testing sites opened in New York City subway stations as several train lines were suspended due to understaffing.

The testing program, which was launched on Monday, originally only included the Grand Central Terminal station and the Times Square-42nd Street subway station -- two of the busiest stations in the city.

However, five more opened Thursday including two in Queens, one in the Bronx, one in Brooklyn and one in Manhattan.

It comes as the Metropolitan Transit Authority announced it is suspending three subway lines due to staffing shortages linked to COVID-19.

"Like everyone in New York, we've been affected by the COVID surge. We're running as much train service as we can with the operators we have available," the MTA said in a statement.

The B and Z subway lines are suspended all day Thursday along with the W line, which was also suspended on Wednesday.

Dr. Anthony Fauci strongly recommended against large New Year's Eve parties at Wednesday's White House briefing.

"Should you change or cancel your plans? If your plans are to go to a 40-50 person New Year's Eve party with all the bells and whistles and everybody hugging and kissing and wishing each other a happy new year, I would strongly recommend that this year, we do not do that," Fauci said.

Fauci said "all indications point to a lesser severity of omicron versus delta." But he warned, "we should not become complacent since our hospital system could still be stressed in certain areas."

White House COVID-19 coordinator Jeff Zients said, "We have directly helped more than 30 states and territories by deploying over 2,100 federal personnel and thousands of ambulances, ventilators and other critical supplies."

Supplies include gloves, masks, respirators and face shields, he said.

He added, "More than 13,000 National Guard members have been activated in 48 states to support the COVID response, from vaccinations, to testing, to clinical care."

At the briefing, CDC Director Rochelle Walensky defended the agency's shortened isolation guidelines.

"We do know the vast majority of viral transmission happens in those first five days, somewhere in the 85 to 90% range. So if a person can isolate for the first five days they absolutely should," she said. "We also don't know that antigen tests give a good indication of transmissibility at this stage of infection. On the other hand, we know that after five days people are much less likely to transmit the virus and that masking further reduces that risk. And this is why people need to mask for five days after the five days of isolation."

After five days, asymptomatic people with COVID-19 can leave isolation, but must wear masks around others, according to the new guidelines.

-ABC News' Cheyenne Haslett