US health authorities on Thursday urged Americans to avoid cruise travel even if they are vaccinated, citing the surge in Covid-19 cases spurred by the Omicron variant.

"Avoid cruise travel, regardless of vaccination status," said a posting on the US Centers for Disease Control and Prevention's website that upgraded the travel notice on cruising to Level 4, the highest on its Covid risk scale.

"Even fully vaccinated travelers may be at risk for getting and spreading Covid-19 variants," the notice said.

Between December 15 and 29, there were 5,013 Covid cases in US waters reported to the CDC, 31 times the level in the prior two weeks, according to an email from a CDC spokesperson.

The agency recommends mitigation steps that include getting vaccinated and boosted, wearing a mask in indoor settings and taking a test before gathering.

"The virus that causes Covid-19 spreads easily between people in close quarters on board ships, and the chance of getting Covid-19 on cruise ships is very high, even if you are fully vaccinated and have received a Covid-19 vaccine booster dose," the CDC said.

The surge in fresh cases linked to the highly-transmissible Omicron variant has raised fresh challenges for the cruise industry, which resumed service in the United States this summer after being essentially dormant for more than a year.

On Thursday, Royal Caribbean modified or cancelled 16 destinations out of 331 due to the uptick in Covid cases. Shares of Royal Caribbean dipped 0.2 percent in afternoon trading.

Shares of Carnival fell 0.6 percent in early afternoon trading, while Norwegian Cruise Line dropped 1.5 percent.