People stand on a pier outside the Ruby Princess cruise ship and wait to be picked up in San Francisco [Source: AP]

The U.S. Centers for Disease Control and Prevention is investigating a cruise ship that docked in San Francisco on Thursday after a dozen vaccinated passengers tested positive for coronavirus.

The results followed a random testing of 25% of the passengers aboard the 10-day cruise to Mexico and the affected individuals are either asymptomatic or experiencing mild symptoms, the Port of San Francisco said in a statement.

The news came as the omicron variant drives extremely high infection levels in the cruise industry, prompting the CDC to either investigate or observe nearly 100 vessels as of Thursday. Cruise ships are required to notify the CDC of suspected or confirmed positive cases on board, and any vessel with 0.10% of passenger cases meet the criteria for investigation.

Cruise ships returned to San Francisco in October after a 19-month hiatus. The pause was sparked in March 2020 when the Grand Princess carried people infected with the coronavirus, sending thousands of passengers to quarantine in their cabins as the ship idled off the California coast. The ill-fated cruise captured the world’s attention and made the coronavirus real to millions in the United States.

When business resumed, most cruise lines required adult passengers to show proof of vaccination against COVID-19.

To date 22 cruises that have docked in San Francisco have reported COVID-19 positive passengers or crew. All have complied with CDC and San Francisco Department of Public Health rules and protocols. No reported cases have required hospitalization or medical care.

Due to the surge, San Francisco is also asking cruise ships to test passengers who have not received their second doses or booster shots before letting them come ashore.

“This requirement is stricter than federal and state guidelines and is intended to protect the people of San Francisco,” port officials said.

A spokeswoman for Princess Cruises said the vaccination rate among guests and crews aboard the ships was “approaching 100%.” The passengers who tested positive will either go home or be taken to hotels for quarantine, Negin Kamali said.

Another Ruby Princess cruise ship is set to depart from San Francisco Thursday afternoon for another 10-day cruise to Mexico, she said.