MIAMI (NEXSTAR) — The CDC now recommends that people at risk for severe illness from COVID-19 — whether vaccinated or not — should avoid cruises.

The new guidance, issued on Friday, is specifically aimed at older adults, people with preexisting medical conditions, those with weakened immune systems, or those who are pregnant or recently pregnant.

“The virus that causes COVID-19 spreads easily between people in close quarters aboard ships, and the chance of getting COVID-19 on cruise ships is high,” reads the guidance, in part. The CDC also categorizes cruise ship travel under a Level 3 travel notice (“high level of COVID-19”), urging full vaccination for anyone boarding a ship, and recommending the unvaccinated avoid cruises altogether.

In addition, the agency continues to recommend masks even in “crowded” outdoor settings, including any open-air areas of the ships, or while gathered at transportation hubs, such as boarding areas.  

Friday’s guidance comes amid increasing concern over the spread of the COVID-19 delta variant, with case numbers and hospitalizations rising nationwide. Delta, now the predominant strain in the U.S., is more infectious than the previous dominant strain, and can lead to increased transmissibility to those who are fully vaccinated, leading to breakthrough infections.

The unvaccinated, however, are still much more likely to transmit and contract the delta variant, the CDC notes.

The U.S. cruise industry, meanwhile, has been eager to set sail, with most U.S. cruise lines implementing vaccination requirements of travelers who are eligible. Major cruise companies, including Royal Caribbean, Carnival and Norwegian Cruise Line, are also referring customers to view the CDC’s current guidance before booking travel.