Nearly 90 cruise ships are experiencing COVID outbreaks, according to Centers for Disease Control and Prevention data. The CDC is investigating, or already has, more than 77% of boats operating or planning to operate in U.S. waters—111 ships, in all—owing to positive case numbers aboard, it reports, based on data collected Dec. 27.

Outbreaks aboard ships are surfacing despite rigorous precautions cruise lines have put in place since the CDC lifted its no-sail order at the end of 2020. That order had sought to curtail the hundreds of positive cases that emerged aboard ships early on in the pandemic. Since they began setting sail once again this summer, cruise line companies have mandated negative testing and proof of vaccination, and required that passengers wear masks indoors. 

“Many of our members have announced additional measures in response to the Omicron variant, including strengthening testing, masking, and other requirements, as well as encouraging booster vaccine doses for those eligible,” a spokeswoman for the Cruise Lines International Association, a trade group, told Fortune in an email.

These precautions don’t appear to be stymieing the spread of Omicron, COVID’s latest variant, which has surged on both land and sea. While only early data is available on Omicron, the variant appears to be a more contagious, albeit potentially less fatal, strain of the coronavirus. Cases have spiked globally since the variant was first detected in Botswana and South Africa in mid-November. And in response to positive cases aboard, at least four vessels were turned away from ports in the Americas this week, according to CNN.

Earlier this morning, Sen. Richard Blumenthal (D-Conn.) urged the CDC to take action. “Time for CDC & cruise lines to protect consumers & again pause—docking their ships. Cruises are repeating recent history as petri dishes of COVID infection,” Blumenthal wrote in a tweet this morning.

The CDC says it conducts an investigation aboard ships once certain thresholds are met, such as COVID being detected in more than 0.1% of a ship’s passengers or one or more members of its crew. Royal Caribbean last week diverted a ship from planned stops after 55 passengers or crew members tested positive.

The new outbreaks could likely become another blow to the cruise line industry, which has taken on billions of dollars in debt in the wake of the pandemic and last year’s eight-month no-sail order. 

“Our enhanced health and safety protocols have proven to be effective in our sailings over the past year since we restarted guest operations. These include vaccinations, extensive testing, masks, and much more,” a Carnival spokesperson said, noting that all of its crew is fully vaccinated and that all of its eligible crew members had received booster shots. “Our top priority is compliance, environmental protection, and the health, safety and well-being of our guests, crew members, shoreside employees, and the people in the communities we visit.”

Representatives of Royal Caribbean, Disney Cruise Line, and Norwegian did not respond immediately to requests for comment.