Vaccine committee says provinces should give AstraZeneca recipients a different vaccine for second dose
National Advisory Committee on Immunization now says a second dose of an mRNA product is 'preferred'
The National Advisory Committee on Immunization (NACI) is recommending the provinces stop administering the AstraZeneca vaccine in most cases — even as booster shots for people who've already received first doses of the product.
NACI said Thursday that AstraZeneca recipients should instead receive a second dose of an mRNA vaccine, like the ones offered by Pfizer and Moderna.
"An mRNA vaccine is now preferred as the second dose for individuals who received a first dose of the AstraZeneca/COVISHIELD vaccine, based on emerging evidence of a potentially better immune response from this mixed vaccine schedule," NACI said in a statement released today. The "mixed vaccine schedule" refers to the practice of using different products for the first and second doses.
The guidance to divert AstraZeneca doses from the supply chain comes weeks after NACI, an independent body composed of volunteer experts, said the AstraZeneca vaccine is not the "preferred" product for first doses given its associated risk of vaccine-induced immune thrombotic thrombocytopenia (VITT) — a condition that causes blood clots combined with low platelets.
NACI said there is also a risk of developing VITT after receiving a second dose of AstraZeneca. The rate of VITT after second doses of AstraZeneca is thought to be lower than the rate for first doses, but NACI said the likelihood of developing this condition after getting a booster shot "has increased over time, with current estimates of approximately 1 per 600,000 people vaccinated."
While NACI initially said Canadians could opt for a viral vector shot like AstraZeneca if they didn't want to wait for an mRNA vaccine, the committee said today that the guidance has since "evolved" and a Pfizer or Moderna shot "should be offered to start a vaccine series, unless there is a contraindication, for example an allergy to one of the mRNA vaccine ingredients."
NACI said provinces can easily shift to an mRNA-based immunization campaign because Canada is poised to receive a flood of shots from Pfizer and Moderna in the coming weeks, with 14 million more doses set to arrive this month.
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Since early May, all provinces have halted the use of AstraZeneca for first doses. Many have continued using it for booster shots.
NACI said people who already have had two doses of AstraZeneca "can rest assured that the vaccine provides good protection against infection and very good protection against severe disease and hospitalization."
Dr. Donald Vinh, an associate professor of medicine at McGill University and an expert in microbiology and human genetics, said Canadians who already have received an AstraZeneca shot should know that NACI isn't "blacklisting" the shot — they're simply favouring highly effective alternatives that are more readily available.
"It's not as if there's been a recall on this product because it's dangerous. There isn't anything inherently wrong with the vaccine. It's not something evil. We're just prioritizing the vaccines that are best suited for the current variants," he said.
NACI said evidence emerging from studies in Germany suggests mixing one dose of AstraZeneca with a second dose of a Pfizer vaccine can actually produce "a potentially better immune response, including against variants of concern" than two doses of AstraZeneca alone.
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"Evidence continues to suggest a first dose of the AstraZeneca vaccine followed by a second dose of the Pfizer-BioNTech mRNA vaccine has a good safety profile," NACI said.
A single dose of the AstraZeneca or Pfizer vaccines has been found to generate a significant antibody response to the novel coronavirus.
But a recent study by the U.K. Coronavirus Immunology Consortium and the University of Birmingham found that the AstraZeneca vaccine may actually induce a stronger cellular immune response than the Pfizer shot.
Dr. Theresa Tam, Canada's chief public health officer, said that, based on early data, a mix-and-match regimen has "performed very well and seems to have a superior response" as it produces both a strong antibody and cellular response.
NACI is also now recommending that mRNA recipients receive a second shot of whatever product they received first.
The second shot should match the first unless "the same product is not readily available, or the product used for the first dose is unknown," NACI said. The two mRNA products can be considered "interchangeable," the committee said.
Brig.-Gen. Krista Brodie, the military commander in charge of vaccine logistics, said Canada will receive about 9.5 million doses of Moderna by month's end — roughly 1.5 million more than the government had projected would arrive.
That 9.5 million figure includes the one million doses that were donated to Canada by the U.S. this morning.
"This is the big lift," Brodie said. "Vaccine supply is rapidly increasing."
While the Moderna supply is stabilizing after months of uncertainty and slashed deliveries, Pfizer has made some adjustments to its shipping schedule for July.
Brodie said Pfizer is still planning to meet its commitment to send nine million doses to Canada next month but the government now expects "a lower allocation in early July and a larger allocation later in July to offset."