(CNN) – At least four ocean cruise ships were turned away this week due to Covid-19 cases on board of ports of call or were not allowed to let passengers off board in America.

Although there have been cases on other cruises since US-based ships Resumption of service this summer Given the need for vaccinations and other measures to minimize outbreaks, the number of cruises forced to change their itineraries appears to have increased.The disruptions are still a long way off March 2020when the pandemic crippled the industry and led to weeks of efforts to get already embarked passengers and crew home as ports were closed to virus-infected ships.Still, this week’s disruption came amid global concerns about the highly transmissible variant of Omicron coronavirus, and there Covid-19 case rates increased in the US and elsewhere.

Recent disruptions on cruise lines include:

• Mexican officials prevented that Holland America line King dam disembark its passengers in Puerto Vallarta on Thursday after 21 crew members tested positive for Covid-19, the Jalisco State Health Department said.

A “small number of fully vaccinated crew members” tested positive and all showed either mild or no symptoms, Holland America Line told CNN.

The Koningsdam.

Alfredo Martinez / Getty Images

Jalisco health authorities initially wanted people who tested negative to disembark. They changed their minds, however, and led an “exponential growth in confirmed cases in the crew” through Thursday, the department said. Only one crew member had tested positive less than a week earlier, the department said.

The ship, which left San Diego on December 19 with more than 1,000 passengers and more than 870 crew members, visited Cabo San Lucas and Mazatlán before arriving in Puerto Vallarta. It should return to San Diego on Sunday as planned.

Holland America Line, like the other cruise lines in this story, requires passengers and crew to be fully vaccinated against Covid-19, with a few exceptions for children.

• Ports on the Caribbean islands of Bonaire and Aruba turned away that Carnival freedom Ship on Wednesday or Thursday after “a small number” of people on board tested positive for Covid-19, the cruise company said.

However, the company made a replacement stop in the Dominican Republic on Friday.

It was allowed in Curacao. stop on Tuesday. Dr. Izzy Gerstenbluth, the national epidemiologist for Curacao, said the ship had notified him prior to arrival that crew members had tested positive. After visiting the ship and finding that the cases were trapped in the crew, Curacao allowed the ship’s passengers to disembark, with the crew remaining on board, he said.

The ship returned to Miami on Sunday as planned, the cruise company said.

Passenger Jim Storupski told CNN subsidiary WPLG his experience on board was positive, adding that the company was “very accommodating”. His wife Connie praised the company for its Covid-19 safety protocols and said they “had a good time”. The couple said they plan to sail with the company again in two weeks.

Other passengers reported different experiences. “It was awful,” Leah Murray said WPLG their frustration with the Covid-19 cases on board. “You have given us little information. You are all slacking off your business.” She said she thought the line with the Covid-19 protocols was “very irresponsible”.The Royal Caribbean Odyssey of The Seas.

The Royal Caribbean Odyssey of The Seas.

Joe Raedle / Getty Images

• Officials in Curacao and Aruba rejected the Royal Caribbean Odyssey of the Seas this week after 55 fully vaccinated crew members and passengers contracted Covid-19, the Miami Herald reported Reported Wednesday.

Curacao health officials ruled the percentage of people who tested positive on board was too high for the nation to berth the ship, the Curacao Chronicle reported.

The ship is scheduled to return to Florida’s Fort Lauderdale on Sunday.

According to Royal Caribbean, the ship can accommodate up to 5,550 guests plus 1,600 crew members. It was not clear whether the ship was fully occupied.

The Seven Seas Mariner.

The Seven Seas Mariner.

Manfred Segerer / ullstein bild / Getty Images

• Colombian officials prevented that Seven Seas Mariner Do not let anyone alight in Cartagena on Wednesday after seven positive Covid-19 cases – including six crew members and one passenger – were reported. The ship started and 18-day trip from Miami to San Francisco on December 18

So far, only a small proportion of the cruise ships in operation have been affected by the disruptions

The changed cruises make up a small part of that Dozens of cruise ships on the move in the Caribbean Sea, the Gulf of Mexico, the wider Atlantic and the Pacific at some point this month.

Positive Covid-19 tests on board cruise lines are not unique this month – they have occurred at different times since cruises resumed from US shores this summer.

For example, in August 27 people tested positive on a Carnival cruise ship before calling at a port in Belize City.

Now, the proliferation of the Omicron variant “can affect the way some target authorities themselves view a small number of cases, even when handled with our strict protocols,” Mardi Gras spokeswoman AnneMarie Mathews told CNN on Friday in a prepared statement .

“Some destinations have limited medical resources and focus on managing their own local response to the variant,” added Mathews.

“Should a port have to be canceled, we will do our best to find an alternative destination,” said Mathews.

Health and Safety Protocols

The companies recommend booster shots to their passengers. Holland America Line says its crew members will be given boosters as soon as they are eligible.

Cruise companies generally require their passengers to test negative for Covid-19 prior to travel and require their crews to be tested frequently.

Passengers who tested positive during the trip are generally required to isolate themselves

US-based cruises also require the wearing of masks in indoor public areas. This policy was recently updated extend the use of indoor masks to fully vaccinated passengers.

CNN’s Francesca Street, Melissa Alonso, Aya Elamroussi, Carma Hassan, Hira Humayun, Stefano Pozzebon, Valentina DiDonato and Sharif Paget contributed to this report.