A dozen passengers on a 10-day Mexican cruise tested positive for the coronavirus just before the ship docked in San Francisco on Thursday.
The passengers, whose infections were discovered through random testing, were either asymptomatic or had mild symptoms, according to the Port of San Francisco.
They either rode home in private vehicles or were quarantining in hotels, Princess Cruises said in a statement.
It was unclear whether all of the cruise's other passengers were tested or whether they were allowed to freely disperse after the positive results emerged.
All aboard were required to be vaccinated and show proof of a negative test before embarking on the cruise, according to port officials.
After being briefly "under observation" by the US Centers for Disease Control and Prevention, the Ruby Princess left for another Mexico cruise on Friday.
Like the rest of the country, San Francisco is seeing a surge in new cases due to the highly contagious Omicron variant.
Community transmission across the nation is at its highest levels since the start of the pandemic, with over 700,000 new cases reported as of Thursday, according to the CDC.
After a 19-month pause due to the pandemic, cruises resumed in the US last summer and in San Francisco last October, with all passengers and crew required to be fully vaccinated, except for medical exemptions allowed by some companies.
Starting Friday, anyone on a ship entering the Port of San Francisco who has not received the full course of COVID-19 vaccinations, including a booster, must be tested before coming ashore.
In early 2020, when COVID deaths were still mostly confined to China, an outbreak aboard the Grand Princess cruise ship killed one person and left thousands of passengers and crew members confined on board at the San Francisco port.
More than 100 passengers ended up testing positive, and 850 had to quarantine at an air force base for nearly three weeks.