A behind-the-scenes look at why Canada delayed 2nd doses of COVID-19 vaccines

Experts say efficacy of 1st COVID-19 vaccine dose is a ‘game changer,’ but critics call delay an ‘experiment’

Adam Miller · CBC News ·  March 6

Canada is the only country in the world delaying second doses of COVID-19 vaccines to four months. Critics say we are venturing into uncharted scientific waters that may lead to complications. (Evan Mitsui/CBC)


This is an excerpt from Second Opinion, a weekly roundup of health and medical science news emailed to subscribers every Saturday morning. If you haven’t subscribed yet, you can do that by clicking here.

Danuta Skowronski was poring over Pfizer-BioNTech vaccine data on a Friday night in mid-December when she had an “aha!” moment.

The epidemiology lead at the British Columbia Centre for Disease Control realized she could actually “correct” the data Pfizer had submitted to the U.S. Food and Drug Administration on the effectiveness of just one dose of its vaccine.

In clinical trials, Pfizer couldn’t accurately determine the efficacy of a single shot because participants had already received their second dose after three weeks, and there was no comparative one-dose study done.

Pfizer reported an efficacy of 52 per cent for one shot, compared to the more commonly cited 95 per cent after the second. 

But Skowronski, who has been working on vaccine effectiveness analyses for more than 15 years, realized the company had included in its analysis the two-week time period immediately after vaccination — before the body’s immune response typically kicks in.

Dr. Danuta Skowronski with the B.C. Centre for Disease Control is in favour of delaying second doses to four months after analyzing the data Pfizer submitted to the U.S. Food and Drug Administration.(Harman/CBC)



She told CBC News vaccines are never expected to protect “instantaneously,” and that there is always a “grace period” of a couple of weeks that factors into vaccine effectiveness.

“What we found was that they were underestimating the efficacy of the first dose, and rather than the efficacy being 52 per cent, it was actually 92 per cent,” she said. “For us, that was a game changer.”

The finding led the National Advisory Committee on Immunization (NACI) to change the recommended time between doses of COVID-19 vaccines from three weeks to an unprecedented four months.

B.C. announced it would be delaying second doses earlier this week. Ontario, Quebec, Alberta, Manitoba and Newfoundland and Labrador quickly followed suit.

Canada is now an outlier in the global vaccination rollout. No other country in the world has delayed second doses up to four months, and there is no evidence yet on the long-term effect it could have on immunity to COVID-19. 

Some scientists say we are venturing into uncharted waters. Others are comfortable with the risk.

To read more, please click on: A behind-the-scenes look at why Canada delayed 2nd doses of COVID-19 vaccines