TORONTO -- The National Advisory Committee on Immunization (NACI) is now recommending that some immunocompromised people receive three doses of a COVID-19 vaccine.

The latest guidance says that “moderately to severely immunocompromised” individuals who have not yet been immunized should receive three doses of an authorized mRNA vaccine. Those who have already completed a series of an mRNA or a viral vector vaccine should get an additional dose of an mRNA vaccine, NACI recommends.

Two mRNA vaccines, manufactured by Pfizer and Moderna, have been authorized for use in Canada.

NACI Chair Dr. Shelley Deeks said in a statement Friday that the recommendation is being made after a careful review of available data.

“This is not unusual for immunocompromised groups, where we often recommend different vaccine schedules to help them achieve better protection,” Deeks said.  “This is different from a booster dose, which would be used to boost an immune response that has waned over time.  NACI is also looking at whether booster doses might be needed for some key populations, but it is too early to comment on the state of the evidence for general boosters at this time.”

Canada’s Chief Public Health Officer Dr. Theresa Tam said there’s a “very broad” group of individuals who may have an underlying condition or be undergoing medical treatments that compromise their immune systems. Examples include people being treated for solid tumours or blood malignancies and organ transplant recipients who take immunosuppressive drugs, she told reporters at a news conference Friday.

“What the recommendation is, and how that is derived, is really looking at the fact that some of these individuals, through studies, have had a lower immune response to the initial one or two doses of COVID-19 vaccine compared to the general population,” Tam said. She said recent, limited studies show that some of those individuals can have an increased immune response after a third vaccine dose.

Tam said immunocompromised individuals should talk to their health care providers to discuss their particular situations.

The province of Quebec is already offering booster shots for the immunocompromised and for travellers whose mixed dosing isn’t recognized in other countries. Alberta and Ontario are also rolling out third shots for eligible immunocompromised populations.

With files from Brooke Taylor