Public health authorities in the US and Brazil are warning against cruising amid Covid-19 outbreaks on ships.

The Brazilian Health Regulatory Agency (Anvisa) on Sunday called for the "immediate temporary interruption of the cruise ship season in Brazil", according to Reuters.

"In view of recent events, Anvisa does not recommend the embarkation of passengers who have trips scheduled on cruise ships for the next few days," the agency said, according to the newswire.

"This recommendation takes into account the rapid change in the epidemiological scenario, the risk to the health of passengers and the unpredictability of operations at this time."

The 4,363-berth MSC Splendida (built 2009) was prevented from embarking new passengers over the weekend after calling on Santos, while Anvisa ordered a second ship to suspend service and is investigating three others, Reuters reported.

Anvisa's decision followed a move by the US Centers for Disease Control (CDC), which raised the Covid-19 risk for cruise ships from level 3 to 4. The agency said on 30 December that the move reflects the "increases in cases onboard cruise ships since identification of the Omicron variant".

The CDC recommends avoiding cruise travel, regardless of vaccination status, testing both before and after the voyage, and self-quarantining for five days following the cruise.

The Cruise Lines International Association called the warning "perplexing" as Covid-19 cases on board make up a "slim minority" of all passengers.

"The majority of those cases are asymptomatic or mild in nature, posing little to no burden on medical resources onboard or onshore," the industry group said.

"No setting can be immune from this virus — however, it is also the case that cruise provides one of the highest levels of demonstrated mitigation against the virus."

Cruise shares rallied in late December following Carnival's positive forecast for 2022, but were pulled downward after Covid-19 outbreaks began circulating shortly thereafter with more than 90 ships having cases aboard.

On Monday, cruse major shares rose once again, with Norwegian Cruise Line Holdings gaining $1.46 to close at $22.02.

Carnival shares jumped $1.30 to $21.42 and Royal Caribbean Group rose $3.93 to $80.83.