Canada's public health agency says people can mix COVID-19 vaccines if they want to, citing cases where local supply shortages or health concerns might otherwise prevent some from completing their two-dose vaccination regimen.
The new recommendations come after safety concerns were raised linking the AstraZeneca vaccine to the potential for dangerous blood clots — a condition the health agency calls "rare but serious." That vaccine is not authorized for use in the U.S., but the Johnson & Johnson vaccine, which has faced similar scrutiny, is. Both of them are viral vector vaccines.
Several Canadian provinces have recently moved to mix vaccines, the CBC reported, because of supply issues, when the vaccine used for a first dose isn't available for the second.
Public confidence is also an issue: Health officials cite a study from late April that found more than 90% of participants said they were comfortable with either the Pfizer or Moderna vaccines, but only 52% of participants said they were comfortable with the AstraZeneca vaccine. Both Pfizer and Moderna are mRNA vaccines.
Based on the available evidence, "we are recommending that someone who received a first dose of the AstraZeneca ... vaccine may receive an mRNA vaccine for their second dose," said Dr. Caroline Quach-Thanh, who chairs Canada's National Advisory Committee on Immunization.
The agency cites the results of a study in Germany and clinical trials in the U.K. and Spain as supporting the safety of vaccine interchangeability. It says it expects further data from ongoing studies in Canada and elsewhere in the coming months and will update its recommendations if warranted.
Two vaccines are dominant in both the U.S. and Canada
Nationwide, the Pfizer-BioNTech and Moderna vaccines have increasingly become the primary vaccines administered in Canada, according to the latest government data.
The Pfizer-BioNTech and Moderna vaccines "can be considered interchangeable" between the first and second doses, Canada's advisory committee on immunization says in its recommendations that were updated this week.
Canadian officials are hoping the new guidance will help bridge a wide gap in their vaccination program. As of late May, 50.6% of Canada's population had received at least one vaccination shot — but only 4.6% of the population was fully vaccinated.
"This is not a new concept," the advisory group said of the practice known as heterologous vaccination. In the past, the group added, "Different vaccine products have been used to complete a vaccine series for influenza, hepatitis A, and others to complete a vaccine series for influenza, hepatitis A, and others."
Several European countries have already been encouraging people who've received a first shot of the AstraZeneca vaccine to make either the Pfizer or Moderna vaccines their second shot, including France, Spain and Germany.
Two vaccines may be better than one
In addition to potentially easing vaccine reluctance, mixing vaccines could also bring other benefits.
As NPR's Joe Palca reported last month: "Basically, all vaccines work by showing people's immune systems something that looks like an invading virus but really isn't. If the real virus ever comes along, their immune systems will recognize it and be prepared to fight it off.
"Using two different vaccines is a bit like giving the immune system two pictures of the virus, maybe one face-on and one in profile."
As other countries authorize mixing vaccines, the U.S. is not following suit — in part because the Food and Drug Administration hasn't authorized the AstraZeneca vaccine. And unlike that vaccine, the Johnson & Johnson vaccine requires only a single dose.
When asked for comment about the strategy of mixing vaccines, an FDA spokesperson cited a lack of data about the interchangeability of the vaccine with other COVID-19 vaccines.
"Individuals who have received one dose of the Pfizer-BioNTech or Moderna COVID-19 vaccine should receive a second dose of the same vaccine to complete the vaccination series," the spokesperson added.
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