Canada looking at evidence vaccines almost as effective after one dose
Delaying second doses could allow more of the vulnerable to be protected
TORONTO — There is compelling evidence that a single dose of COVID-19 vaccines may provide almost as much protection as giving two doses, Canada’s deputy chief public health officer said Thursday.
Dr. Howard Njoo said the advisory committee of federal and provincial public health officers is having an active discussion about whether Canada would be better served to delay the second doses of vaccines in a bid to give protection to more vulnerable people quicker.
“These are what I would call early data in terms of a vaccine effectiveness or studies,” he said. “And the indications are that there’s a good level of protection after just one dose.”
Canada intends to vaccinate three million people with two doses by the end of March.
More than 990,000 Canadians have received at least one dose, and about one-third of those have also received their second doses.
Quebec’s immunization committee went as far Thursday as to recommend nobody get a second dose until a first dose is injected into everyone in six high-risk groups, including people over 70, health-care workers, and people who live in long-term care homes and retirement residences.
The committee reported that single doses have been 80 per cent effective at preventing COVID-19 so far among longterm care residents and health workers who were vaccinated.
Questions about delaying the second doses arose almost as soon as vaccinations began in December, prompting a whirlwind of debate among scientists about the ethics of “going off label.”
Pfizer-BioNTech and Moderna’s vaccines — the two currently authorized in Canada — were tested by giving two doses, 21 days and 28 days apart respectively.
But Dr. Danuta Skowronski, the epidemiology lead for influenza and emerging respiratory pathogens at the British Columbia Centre for Disease Control, said data show two weeks after one dose of either vaccine, the protection against COVID-19 was almost as good as what was found after two doses.
Skowronski and Dr. Dr. Gaston De Serres from the Institut national de santé publique du Québec made the case in a letter published in the New England Journal of Medicine this week.
Moderna reported itself that two weeks after one dose, those who got the vaccine were 92 per cent less likely to develop COVID-19 symptoms.
Pfizer and BioNTech, whose vaccine uses similar genetic technology to Moderna’s, said their vaccine was 52 per cent effective after one dose, but 94.5 per cent effective after two.
Skowronski said Pfizer started measuring illness as soon as the injections were given, which she said is “unreasonable.”
“It’s basic vaccinology that you don’t expect the vaccine to activate the immune system instantaneously,” she said, in an interview with The Canadian Press.
A new vaccination schedule issued Thursday shows if the other three vaccines currently being reviewed get approved by Health Canada, 24.5 million Canadians could be vaccinated before Canada Day.