Have you sailed recently on one of these 21 cruise ships? You may have been exposed to coronavirus

Tens of thousands of cruisers on voyages involving U.S. ports in recent months may have been exposed to the new coronavirus, according to data released by the U.S. Centers for Disease Control and Prevention (CDC).

A cruise-focused segment of the agency's website updated on Sunday lists 28 sailings on 21 ships in February and March that had passengers on board who contracted the virus.

In some cases, the passengers began experiencing symptoms of COVID-19 while still on board the vessels. In other cases, they began exhibiting symptoms shortly after disembarking.

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The voyages include sailings on three of the world's 10 biggest cruise vessels: Royal Caribbean's Symphony of the Seas and Oasis of the Seas, and MSC Cruises' MSC Meraviglia. All three of the sailings began and ended in Miami.

The CDC lists 11 voyages operated by six lines where passengers began showing symptoms while still on board. These are the vessels and the sailing dates:

• Celebrity Eclipse (March 2 to 30)
• Disney Wonder (March 6 to 20)
• Grand Princess (Feb. 21 to March 7)
• Norwegian Bliss (March 1 to 8)
• Norwegian Breakaway (March 7 to 14)
• Pride of America (Feb. 29 to March 7)
• Riviera (Feb. 26 to March 11)
• Liberty of the Seas (March 15 to 29)
• Majesty of the Seas (Feb. 29 to March 7)
• Oasis of the Seas (March 8 to 15)
• Symphony of the Seas (March 7 to 14)

There were 17 more sailings operated by eight lines where passengers who contracted COVID-19 began showing symptoms shortly after disembarking. These are the ships and the sailing dates (in the case of two ships, more than one sailing was affected):

• Carnival Imagination (March 5 to 8)
• Carnival Valor (Feb. 29 to March 5; March 5 to 9; March 9 to 14)
• Carnival Vista (Feb. 15 to 22; Feb. 29 to March 7)
• Celebrity Infinity (March 5 to 9)
• Celebrity Reflection (March 13 to 17)
• Celebrity Summit (Feb. 29 to March 7)
• Crown Princess (March 6 to 16)
• Disney Wonder (Feb. 28 to March 2)
• Grand Celebration (Feb. 22 to 24)
• Grand Princess (Feb. 11 to 21)
• MSC Meraviglia (March 1 to 8)
• Norwegian Bliss (March 8 to 15)
• Norwegian Breakaway (Feb. 29 to March 7)
• Explorer of the Seas (March 8 to 15)

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For the latter group, passengers showed signs of COVID-19 within 14 days after disembarking -- the estimated maximum incubation period of the illness. That means they could have had the illness while still on board the ships. But the CDC says it's impossible to know for sure.

"The traveler might have contracted COVID-19 during the voyage," the agency said. "However, other sources of transmission after the voyage cannot be ruled out."

Cruisers should note these lists do not include every sailing around the world where passengers may have been exposed to the new coronavirus. The CDC only has jurisdiction over voyages that include at least one stop at a U.S. port, and thus it only tracks illnesses on such sailings. Cruise ships entering U.S. ports are required to inform the CDC of illnesses on board.

Related: Cruise ships could be put into storage for months due to coronavirus

Several high-profile coronavirus outbreaks on cruise ships, such as the February outbreak on the Diamond Princess in Japan, are noticeably absent from the list. That's because the vessels were on itineraries that didn't include a U.S. port. More than 700 passengers on the Diamond Princess tested positive for the coronavirus and 10 died.

Two very recent outbreaks of coronavirus -- on Holland America's Zaandam and Princess Cruises' Coral Princess -- are not on the list because the ships only touched a U.S. port in the last few days.

Related: Death toll rises as Princess tries to clear cruise ship experiencing outbreak

The 28 sailings listed by the CDC could have been carrying as many as 86,658 passengers, according to a TPG analysis of berth availability on the ships.

The listings appear at a section of the CDC website that explains a new coronavirus outbreak-related directive that tightens the requirements for cruise lines disembarking passengers from ships in U.S. ports.

On Saturday, the CDC said cruise lines no longer could use commercial flights or other forms of public transit to transport passengers home from ships. Instead, the agency said lines must charter private aircraft or use other private transportation methods such as private cars to get passengers home.

The directive applies to all travelers arriving in the U.S. via a cruise ship — not just those experiencing symptoms of COVID-19.

In announcing the directive, the CDC specifically mentioned the high number of cruise ships where passengers may have been exposed to the coronavirus.

"Since February 2020, travelers on dozens of cruise ships have been affected by COVID-19 outbreaks," the agency said. "Cruise ships are often settings for outbreaks of infectious diseases because of the semi-enclosed environment and contact between travelers from many countries. Outbreaks of COVID-19 on cruise ships pose a risk for rapid spread of disease beyond the voyage. Aggressive efforts are required to contain spread."

As part of the new directive, the CDC said passengers leaving cruise ships should wear a face mask or cloth face covering while traveling home from vessels. The agency said cruise lines were responsible for providing such masks to every departing passenger.

Once at home, cruisers should then remain at home for 14 days, the CDC said.

Additional resources for traveling during the coronavirus outbreak:

• How to cancel or postpone a cruise due to coronavirus
• How coronavirus is impacting airline award availability
• How coronavirus has left the travel industry reeling
• Airlines scale back inflight offerings due to coronavirus
• How to ward off coronavirus in your hotel room
• Guide to traveling during the coronavirus outbreak