TORONTO—Health workers and humanitarian groups at more than 200 ports around the world are making a push to vaccinate thousands of mariners, a population of essential workers that has been largely neglected in the fight against Covid-19.

Many of the thousands of global mariners that are unvaccinated have been unable to disembark from their ships and have been stuck aboard, not seeing their families or standing on land for several months. In addition, infections at ports or on ships have disrupted global shipping at a time...

TORONTO—Health workers and humanitarian groups at more than 200 ports around the world are making a push to vaccinate thousands of mariners, a population of essential workers that has been largely neglected in the fight against Covid-19.

Many of the thousands of global mariners that are unvaccinated have been unable to disembark from their ships and have been stuck aboard, not seeing their families or standing on land for several months. In addition, infections at ports or on ships have disrupted global shipping at a time when bottlenecks are already slowing the world’s economic recovery from Covid-19.

Port authorities and nonprofits at some of the world’s busiest ports, including Los Angeles, Rotterdam and Singapore, now offer Covid vaccines to international seafarers. For many of the sailors, it is the first time since the pandemic began that they have been able to get access to vaccines.

The Port of Vancouver is Canada’s busiest port.

Iluminado Jagonos Jr., the 51-year-old captain of the JP Azure, a bulk carrier shipping coal from Vancouver to Boryeong, South Korea, has been stuck on ship three months longer than his eight-month contract, unable to return to his family in the Philippines without a vaccine certificate.

Capt. Jagonos heard about Vancouver’s program, and in October his ship pulled into port and healthcare workers from Vancouver Coastal Health came aboard offering Pfizer shots to the crew of 20. As the captain received his first dose, he led his crew in a cheer to celebrate.

“You feel invincible when you have the vaccine,” said Capt. Jagonos.

There are no official statistics on how many seafarers have been vaccinated in recent months, but thousands of doses are becoming available. The Port of Rotterdam in August announced it was making 10,000 vaccines available for all seagoing vessels.

For the past two months, healthcare workers at Vancouver Coastal Health have been helping to vaccinate crew members aboard ships.

According to the International Chamber of Shipping, a trade group, more than half of the world’s 1.7 million seafarers come from developing nations like the Philippines, Indonesia and India, which have lagged behind in vaccinating citizens.

Once unvaccinated mariners board a ship, they find they can’t take shore leave because of local rules that keep unvaccinated people out, or because their captain and shipowner doesn’t want to risk them contracting Covid-19 and bringing it back with them. That leaves the workers stranded aboard floating steel islands with no reprieve.

“People are sad. We lost the enjoyment of the seaman’s life,” said Daresh Villarayan, 49, a motorman on the chemical tanker, MT Peterpaul. He hasn’t left the boat once since he stepped aboard more than seven months ago.

Seafarers aboard the Kagawa Maru lined up for their Covid-19 vaccines in October when the bulk carrier was in Vancouver.

“We don’t have freedom. It is the same as jail,” he said.

The ship is en route from Changzhou, China, to the United Arab Emirates port of Fujairah.

“It places an enormous strain on the crew,” said Peter Lahay, a coordinator for the International Transport Workers Federation, a global trade union.

The industry is also advocating for vaccines, since the economic impact of an outbreak can be severe. A Covid-19 infection aboard a ship or at a port can quickly ripple across the global supply chain.

In August, a Covid infection caused the shutdown of a key terminal for shipments to Europe and North America at the Ningbo-Zhousan port in China. The congestion it created spread to Shanghai and Hong Kong as other ports struggled to ease the backlog.

In July, a suspected Covid infection aboard the MSC Ines container ship forced the closure of one of three docking berths at the Port of Vancouver, said Bonnie Gee, vice president of the Chamber of Shipping, a Canadian shipping industry group. The quarantined ship remained at the berth for roughly two weeks, reducing traffic by a third at the biggest container terminal in Canada’s busiest port.

The seafarer vaccination efforts first began in the U.S. in April, led by local aid organizations like the North American Maritime Ministry Association, a Christian charity.

JP Azure crew members waited out the 15 minutes after receiving their shots in the ship’s so-called recovery room.

It was difficult at first to convince authorities to help seafarers because they were foreigners at a time when countries were giving priority to domestic populations for vaccines, said Jason Zuidema, executive director of NAMMA.

“We convinced people that if seafarers are not vaccinated, and they have outbreaks, it affects me and you,” said Mr. Zuidema. “Our products can’t make it to us.”

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Ports in Florida and Texas were among the first to begin vaccination programs, he said. Seafarers were given surplus Johnson & Johnson vaccines, because the one-shot regimen meant the sailors wouldn’t have to search for a second dose elsewhere.

At the ports of Los Angeles and Long Beach, the busiest container ship ports in the U.S., roughly 8,500 people have been inoculated since April, said Guy Fox, chairman of the International Seafarers Center of Long Beach/Los Angeles, a nonprofit that provides humanitarian services to mariners. The organization has been working with the ITF union and local health authorities to vaccinate the seafarers.

“The vaccines are important. It’s the least we can do for these workers,” said Mr. Lahay, coordinator of the ITF.

The vaccination team disembarks from the Kagawa Maru to return to shore.

Write to Vipal Monga at vipal.monga@wsj.com