Tens of thousands of seafarers are stuck at sea working beyond their maximum 11-month contracts, as the emergence of the Omicron Covid-19 variant hinders cargo ship operators’ ability to change over crews.

With the world’s 1.5 million sailors spending long stretches far from land, only about a quarter of them are fully vaccinated, ship operators, crewing agencies and industry trade bodies estimate.

The low rate of immunization is constraining the movement of crews as countries enforce new travel restrictions for people who haven’t gotten their shots.

Many ports in Asia require seafarers to wait seven to 10 days before disembarking to ensure they aren’t infected. Meanwhile, sailors traveling to port cities to start contracts are getting blocked at border crossings that require proof of full vaccination.

Crew changes routinely take place in big gateway ports in China, Singapore, Los Angeles, Houston, the Netherlands and Belgium, with more than 150,0000 seafarers flying around the globe each month to connect with ships and replace colleagues who have been at sea.

Governments have been trying to balance efforts to curb the spread of Covid-19 against the need to keep products moving world-wide amid widespread supply-chain disruptions.

Several seafaring countries including the U.S., Indonesia and Greece have said they would recognize sailors as essential workers, but the difficulty of coordinating cross-border efforts has slowed progress, according to a senior Greek government official. Among the hurdles, the official said, is that many ports, airports and other entry points haven’t been clearly instructed to assign priority to sailors, and the sailors haven’t been given proper documentation for priority border checks and boardings.

The International Maritime Organization, the United Nations body that regulates global shipping, has repeatedly said the inability to rotate crews has emerged as the biggest challenge facing maritime operators. It has called on governments to recognize seafarers as essential workers for global supply chains and exempt them from travel barriers.

“They are still deemed not essential personnel, but without those workers you would get no food, medicine and other essentials,” said Jan Dieleman, president of Cargill Ocean Transportation, which that operates more than 700 ships and is a unit of American commodities giant Cargill Inc. “Sailors can’t travel or get priority for vaccines. It’s super complicated.”

The peak for stranded seafarers on ships came in late 2020 when nearly 400,000 had to work for months beyond their contracts, according to the International Chamber of Shipping, an industry trade body. The ICS, along with ship operators and crew managers, pegged the current number of stranded sailors at about 200,000.