COVID-19 booster shots recommended for high-risk Canadians starting this spring

Canada's national vaccination advisory body is calling for high-risk individuals to get another COVID-19 booster shot, starting this spring.

The new recommendations from the National Advisory Committee on Immunization (NACI) outline that an additional vaccine dose may be offered at a standard interval — six or more months from the last COVID-19 vaccine dose or SARS-CoV-2 infection, whichever is longer — for people at a higher risk of severe illness.

That includes:

• Adult residents of long-term care homes and other congregate living settings for seniors or those with complex medical care needs.
• Adults 18 and up who are moderately to severely immunocompromised, either due to a medical treatment or underlying health condition.
• Adults 65 to 79, particularly if they don't have a known prior history of SARS-CoV-2 infection, plus anyone 80 and up.

Bivalent, Omicron-containing, mRNA-based COVID-19 vaccines are the "preferred" option for booster shots, NACI wrote.

At this time, the committee isn't recommending an additional spring booster for people in the general population who've already received all their previous recommended doses, Chief Public Health Officer Dr. Theresa Tam wrote in a statement on social media.

"Anyone who has not yet received a primary series or recommended fall 2022 booster can get it now to reduce their risk of severe illness from COVID-19," she said.

NACI will, however, "continue to monitor the SARS-CoV-2 epidemiology and emerging evidence, including duration of vaccine protection from bivalent booster doses in the coming months to provide recommendations on the timing of subsequent booster doses if warranted," the committee said in its latest guidance.

Amount of time since last dose matters

As for the six-month interval timing, NACI stressed that evidence shows "the antibody response is higher with longer intervals between infection and vaccination and with longer intervals between vaccination doses," though there is no safety risk associated with shorter intervals. 

It's become clear recently that "time since last vaccination" is the most important determinant of protection against infection, more so than number of vaccinations, explained McMaster University immunologist Matthew Miller in an email exchange with CBC News. 

"For the average individual, the vaccines continue to do an excellent job preventing severe illness even when protection against infection starts to wane," he wrote.

"However, if a high-risk individual were to get infected, they remain at disproportionately high risk of more severe outcomes. Thus, optimizing protection from infection in that group seems prudent."