SAN FRANCISCO (AP) — The 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 Port of San Francisco said 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 news come as the omicron variant drives extremely high infection levels in the cruise industry, prompting the CDC to either investigate or observe nearly 100 vessels.

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