Canada's chief public health officer is warning travellers, especially the elderly and those with fragile health, to reconsider going on cruises after cases of the coronavirus were confirmed on a cruise ship which has Canadians on board.

"Think twice about going on cruise ships," said Dr. Theresa Tam on Friday, noting they "present environments where COVID-19 can spread easily given close contacts between passengers and crew for significant periods of time."

Tam said that even if people on a cruise do not contract the virus they could be quarantined by destination countries for extended periods of time, increasing the risk of infection.

State officials on Wednesday quarantined the Grand Princess cruise ship off the coast of California over fears of a possible outbreak. There are some 235 Canadians among the 3,500 passengers and crew on board.

According to a statement from the cruise line, fewer than 100 guests and crew members were initially identified for testing by the U.S. Centers for Disease Control and Prevention. Test kits were delivered by air to the ship Thursday.

On Friday, U.S. Vice-President Mike Pence said that so far 46 people have been tested and 21 came back positive; 19 crew and two passengers. It remains unclear if any Canadians tested positive.

Pence said that the Grand Princess will be moved to a non-commercial port where all 3,500 people on board will be tested.

The ship was quarantined after a traveller from its previous voyage — a Feb. 11-21 trip to Hawaii — died of COVID-19 and at least two others became infected.

Tam said there were 260 Canadians aboard that voyage and that the Public Health Agency of Canada has their names. That list, Tam said, was passed to provincial health authorities who are following up with those who have returned home.

Tam also said that so far three of Canada's 54 confirmed or presumptive cases of coronavirus were travellers on Grand Princess' trip to Hawaii.

Another cruise ship, the Diamond Princess, was quarantined by Japanese officials last month because of a coronavirus outbreak. A passenger on a third ship, the Westerdam, tested positive after the vessel docked in Cambodia.

Canadians being tested

More than 4,500 people have been tested for the virus across Canada. Tam said that all provinces and territories are fully equipped to continue testing and additional help is available from the National Microbiology Laboratory in Winnipeg.

With more than 100,000 cases reported in more than 90 countries, and the situation continuing to change daily, Tam said all March Break travellers should regularly check the federal government's travel advisory for their destination country because information on destination countries is constantly changing.

"With so many countries reporting cases of COVID-19, all travellers have to be very vigilant in closely monitoring their health for 14 days after entering Canada. At the first sign of even mild symptoms; stay home," Tam said.

If those symptoms develop, Tam says that people should call their local Public Health Office so they can determine if testing for the virus is required.