People who travel outside Canada, the continental U.S. and Europe won't be able to donate blood for 21 days after their return, Canadian Blood Services says.

The eligibility criteria will change to include a 21-day waiting period to mitigate the risk of Zika virus entering the country's blood system.

The risk of a donor transmitting the Zika virus to a blood recipient is very low, Canadian Blood Services says. (Michael Buholzer/Reuters)

The temporary deferral will safeguard the blood supply against the Zika virus and other mosquito-borne viruses, said Dr. Dana Devine, chief medical and scientific officer of Canadian Blood Services.

"We've now determined that a 21-day deferral period of time for donors to refrain from donating blood after they've returned from travel to a Zika-risk area," Devine said in an interview Wednesday.

The length of time is enough to ensure the virus is no longer incubating and is cleared from a person's system, Devine added.

It starts the day someone returns to Canada, the agency said.

The waiting period will take effect in all clinics starting Friday.

Cord blood and stem cell donors who've travelled to affected areas will face the same waiting period.

The risk of a donor transmitting the Zika virus to a blood recipient is very low, the agency said.

The mosquito that carries the virus does not live in Canada's cold climate.

There have been very few reported cases of Zika virus infection in travellers who acquired the virus abroad.

Héma-Québec, Quebec's blood operator, will adopt the same change.

Canadian Blood Services asks people to donate before they travel to help make up for an anticipated shortfall.

On Thursday, Brazilian health authorities confirmed a case of transmission of Zika through a transfusion of blood from a donor.