Princess Cruises' Grand Princess is staying off the coast of San Francisco after 21 passengers tested positive for coronavirus.

The Carnival Corp-owned brand confirmed Friday that 19 crewmembers and two guests had the virus following a first-phase health screening of 45 passengers aboard the vessel.

Carnival said it has tentative plans for the 107,500-gt, 1998-built ship to come ashore and disembark passengers Monday in Oakland, California, at a time to be determined.

US vice president Mike Pence and the US Centers of Disease Control informed Princess Cruises of the 21 infected passengers during a Friday press conference. Pence earlier today met with cruise executives to discuss the virus outbreak.

Number of passengers to be screened unknown

Princess Cruises does not know how many more of the 2,422 guests and 1,111 crewmembers will be screened for Covid-19, according to Carnival chief medical officer Dr Grant Tarling.

"We're in contact regularly with the US Centers for Disease Control and we have not been informed as of yet what the testing protocol will be," he said during a press conference held Saturday.

"We don't know how many people will need to be tested.

The Grand Princess has remained offshore since Wednesday when it was discovered that two passengers on a previous Grand Princess cruise tested positive for the virus after disembarking from the ship in San Francisco on 21 February.

One man, a 71-year-old California resident, has since died, prompting California governor Gavin Newsom to declare a state of emergency.

Princess Cruises said all guests and infected crewmembers currently onboard the ship will remain isolated in their rooms while the ship's doctor informs them of test results.

"Princess Cruises will continue to closely follow the guidance of the CDC and other federal and state government authorities," the company said.

Guests have been given free internet and telephone access to stay in contact with families and loved ones, as well as free room service while under quarantine on the ship.

The ship's medical team is collecting guest information regarding medication refill needs.

"We are working with CDC to determine what additional precautionary measures, if any, will be required," Princess Cruises said.