While some of the world’s largest cruise lines are scrambling to manage coronavirus outbreaks onboard, at least two companies are offloading crew members who test positive onto ships that are sailing without any passengers.

Two industry giants, Carnival Cruise Line and Royal Caribbean International, say they are transferring workers to crew-only ships to wait out their isolation periods. The movement of crews has raised eyebrows among some passengers, who have documented transfers of more than 100 workers. Neither cruise line would disclose to The Washington Post how many employees are staying on quarantine ships.

“To keep our crew and ships as healthy as possible, we have been using out-of-service ships for our crew members who are asymptomatic and mildly symptomatic, and in isolation after testing positive for COVID-19,” Royal Caribbean spokeswoman Lyan Sierra-Caro said in an email.

Three other major cruise lines did not immediately respond when asked if they were employing similar practices. One passenger on a P & O Cruises ship in the Caribbean told The Post last week that he and his wife were being moved to a ship operated by Cunard Line after testing positive for the novel coronavirus. Both companies are owned by Carnival Corp.

Four Royal Caribbean ships are being used as quarantine vessels for crew members: Vision of the Seas, Rhapsody of the Seas, Serenade of the Seas and Jewel of the Seas. Carnival is using at least two ships: Carnival Ecstasy and Carnival Sensation.

This past week, Royal Caribbean canceled sailings for three ships — Serenade, Jewel and Symphony of the Seas — for periods ranging from a few weeks to a couple of months; the return of Vision of the Seas to regular cruising was postponed to March 7.

Royal Caribbean’s Sierra-Caro said the employees who tested positive are monitored by the ships’ medical team during the course of their 10-day quarantine and then return to their assigned ships. She said this week that no crew members have had serious symptoms or needed to be hospitalized.

Chris Chiames, a spokesman for Carnival, said in an email that “virtually all crew” who are quarantined do not require medical attention.

“But there are physicians and medical staff on board all ships for immediate care, and then we would always transport to a hospital in an emergency,” he said.

The Centers for Disease Control and Prevention, which has allowed ships to operate under a “conditional sailing order” since last year, allows cruises to run dedicated quarantine ships. Spokeswoman Caitlin Shockey said in an email that operators need to isolate confirmed cases in single-occupancy cabins with private bathrooms.

“If the ship does not have enough space to isolate and quarantine their crew, they should have a protocol for managing these cases, which might include transferring crew to offshore locations or other ships,” she wrote.

A passenger on Harmony of the Seas whose young daughter tested positive for the coronavirus posted a video on TikTok under the name “Cruising With Covid” and said that more than 100 crew members were transferred. When someone replied with an allegation that it was fake, the passenger included more of his footage — set to the theme of “Titanic.”

Some industry critics say the transfers amount to a shell game that allows cruise lines to keep numbers lower on ships that are visiting ports — making it more likely those destinations will allow the ships to dock.

“There’s no question in my mind that this started because they wanted to keep their ships operational and going in and out of the ports of call,” said Jim Walker, a Miami-area lawyer who sues cruise lines — often on behalf of crew members — and runs the Cruise Law News website.

He has written about the “plague ships,” a term he says crew members have used with him, several times on the website and wondered whether the ships have adequate medical staffing to handle the patients. Walker reported more than 3,700 infected crew members between three of the ships, but that number could not be independently verified. Royal Caribbean declined to provide numbers or address that report despite several requests.

Walker said he suspects that the cruise lines are also trying to save money by housing some coronavirus-positive crew members on ships instead of in hotels on land and are avoiding what could be more expensive medical care in the United States. The cruise companies did not provide a statement on those allegations, but Chiames said crew members might not be authorized to stay in hotels in some locations.

As numbers of positive cases have soared in recent weeks, some destinations have refused to let cruise liners dock with infected passengers or crew members.

“Decisions by local destinations to deny entry can be somewhat arbitrary based on both the local public health situation in combination with the status of cases on board,” Chiames said.

Sierra-Caro said Royal Caribbean works in partnership with each destination its ships visit and provides a medical declaration with any positive cases on board before arriving.

A former Royal Caribbean crew member, who resigned when informed he would be moved to a quarantine ship after testing positive for the coronavirus in late December, said he believed the practice was an effort to “massage” covid numbers, although he never heard the company say that explicitly.

“They’ve got a covid problem,” he said. “But they’re manipulating the numbers to make it look like they don’t have a covid problem.”

The former crew member, who spoke on the condition of anonymity out of concern about future job prospects, said the transfers were just one issue plaguing ships’ crews. He said he had interacted with passengers who were not properly using their masks and witnessed inconsistency from his employer in applying quarantine requirements. He said that one close contact, a supervisor, did not quarantine after he tested positive.

“The morale is just terrible,” he said.

Sierra-Caro said that Royal Caribbean is operating with an “enhanced face mask policy” and that the operator’s “detailed action plan” calls for close contacts of anyone who tests positive to quarantine in their stateroom for 24 hours before being tested for the virus.

Despite a thirty-onefold increase in cases on cruise ships in the past two weeks of December compared to the previous two weeks, the CDC plans to let its restrictions on sailing expire Saturday. At that point, the rules will become recommendations for cruise lines to follow voluntarily.

The agency recently warned against cruise travel, even for those who have been vaccinated.

CDC Director Rochelle Walensky said during a Senate hearing Tuesday on the response to the omicron variant that the agency would continue to provide oversight, technical assistance and support for ships following the order. She praised the industry’s collaboration with the CDC.

“We anticipate that this order will not be renewed and that the cruise ship industries will continue to understand that this is a really safe practice for those industries,” she said. “What I can’t predict is what the summer will bring.”

Lena H. Sun contributed to this report.