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.

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