The U.S. Centers for Disease Control and Protection identified 89 cruise ships with coronavirus cases Tuesday, nearly all of which have met the threshold for a formal investigation.

Sen. Richard Blumenthal, D-Conn., urged cruise companies and health agencies to stop ships from sailing, saying they are “repeating recent history as petri dishes of COVID-19 infection.” Operators such as Carnival, which owns Seattle-headquartered Holland America Line, have implemented safeguards including mandatory masking and proof of vaccination. Still, the fast-spreading omicron variant has triggered an increasing number of infections and reports of ships being turned away at ports.

“Time for CDC & cruise lines to protect consumers & again pause — docking their ships,” Blumenthal said in a tweet.

Roger Frizzell, a spokesperson for Carnival, said in an email that “our health and safety protocols put in place have proven to be effective time and time again over the past year with our sailings being restarted across each of our brands.”

Of the 86 cruise ships under investigation by the CDC, Carnival operates 32, Royal Caribbean Cruises operates 25, and Norwegian Cruise Line Holdings operates 15. Four ships operated by Disney Cruise Line are now also under watch, according to the CDC website. The list is rapidly changing, and the next stage for ships with outbreaks could involve taking additional public health measures.

“Health and safety is the cruise industry’s highest priority,” Bari Golin-Blaugrund, a spokesperson for the Cruise Lines International Association trade group, said in an email. “In fact, the latest data show that cases have occurred less frequently on cruise ships than on land, with a greater proportion of asymptomatic or mild cases.”

Shares of cruise operators rebounded early Tuesday along with the travel sector, but finished the day down. Carnival was down 0.29% at market’s close, while Royal Caribbean lost 0.34% and Norwegian fell 1.04%. Of the three, only Royal Caribbean shares have had positive returns this year and they have significantly trailed the broader market, up less than 6% year to date compared to a 27% gain in the S&P 500 Index.