Canada's National Advisory Committee on Immunization (NACI) has changed its guidelines to allow for mixing and matching COVID-19 vaccines.
Federal health officials announced the updated guidance during a press briefing on Tuesday, saying that a first dose of the AstraZeneca-Oxford/COVISHIELD vaccine can be followed up with a second AstraZeneca shot, or be safely combined with a second dose of the Pfizer-BioNTech or Moderna shots "unless contraindicated."
The new guidance from NACI also advises that the Pfizer and Moderna vaccines can be mixed for first and second doses.
"NACI has worked to quickly adapt this guidance on the use of COVID-19 vaccines in Canada to ensure optimal protection of Canadians at pace with the ever changing circumstances during this pandemic," Canada's Chief Public Health Officer Dr. Theresa Tam said during the briefing.
"This advice provides provinces and territories with safe and effective options to manage the vaccine programs," she added.
There are currently four vaccines authorized for emergency use in Canada. However, several provinces have suspended the use of the AstraZeneca/COVISHIELD vaccine for first doses over concerns about the risk of developing rare blood clots.
The update to the guidelines follows moves by some provinces to combine different shots due to issues with the AstraZeneca shot. Manitoba health officials announced on Monday that people who received AstraZeneca as their first dose can get a second dose of either Pfizer or Moderna if they meet provincial eligibility.
Prince Edward Island announced on Tuesday that it plans to give those who received AstraZeneca a choice of whether they want AstraZeneca as their second dose or one of the mRNA vaccine options.
It is unclear how soon other provincial health authorities will be able to adjust their vaccine rollouts to take into account NACI's revision for mixing and matching vaccine doses.
While NACI is advising that the vaccines can be safely used in combination, Tam said Canadians should try to get the same dose in a series if possible, specifically when receiving Pfizer or Moderna.
"NACI is not recommending just use them interchangeably, but mostly try and use the same vaccine -- the mRNA -- for the second dose," Tam said. "If you can't find it, or you don't know what someone had, whether it's Pfizer or Moderna for the first dose, another vaccine can be considered."
"Try and find the same vaccine… but if you can't for some reason, then consider them interchangeable," she added.
NACI said "no data currently exist on the interchangeability of COVID-19 mRNA vaccines," but made the recommendation based on the similarity of mRNA technology between Pfizer and Moderna.
"At this time, there is no reason to believe that mRNA vaccine series completion with a different authorized mRNA vaccine product would result in any additional safety issues or deficiency in protection," NACI said in a statement on the revised guidelines.
NACI's previous guidelines advised that the vaccination series should be completed with the same COVID-19 vaccine product, meaning those who received AstraZeneca should follow up with the same shot.
The previous guidance also said that mRNA vaccines should only be used interchangeably if the same first dose is "temporarily unavailable or inaccessible."
Tam stressed that mixing and matching vaccines "is not a new concept." She explained that this has happened before with other vaccines in Canada, including the seasonal flu shot and hepatitis A vaccine, when vaccine programs and suppliers change.
"This is not an unusual thing to do," she said.
Health Canada’s approval for two doses of the same shot was after extensive clinical trials. Tam said NACI's updated guidance on mixing vaccines is based on a "range of factors" including emerging research, the safety and efficacy of the approach, the risk of severe blood clots associated with the AstraZeneca vaccine, and Canada’s vaccine supply.
Tam said NACI also took into account preliminary study results from Spain and the United Kingdom, which found earlier in May that mixing and matching AstraZeneca and Pfizer vaccines was ultimately safe.
Both studies have also found that mixing the two vaccines was effective at preventing COVID-19. However, the studies noted that mixing vaccines resulted in more frequent side-effects within 48 hours of receiving the second dose, including fever, headaches and fatigue, when compared to "non-mixed" schedules.
The U.K. study looked at more than 800 adults aged 50 and over who had received mixed vaccine doses at a four-week interval. The early results were described on May 12 in a research letter in peer-reviewed medical journal The Lancet.
The study found that with the two mixed schedules— Pfizer followed by AstraZeneca, and AstraZeneca followed by Pfizer — both were safe, but resulted in more frequent side-effects after the second dose. Data on efficacy of these mixing schedules is expected to arrive this month.
Those on a mixed schedule who received AstraZeneca as their second booster shot experienced slightly higher levels of side effects than those who received Pfizer as their second booster shot, according to the U.K. study, with 47 per cent of those who received AstraZeneca after Pfizer reporting that they felt feverish, compared to 34 per cent of those who received Pfizer after AstraZeneca.
Dr. Dale Kalina, infectious disease physician at Joseph Brant Hospital in Burlington, Ont., told CTV News Channel that the risk of side-effects shouldn't deter Canadians from getting different vaccine doses.
He explained on Tuesday that the side-effects are typically short lived, only lasting for a few days.
"It's somebody's immune system working against the vaccine, which is exactly what it should be doing. So while there were more side-effects when you mixed a mRNA vaccine with the AstraZeneca vaccine, I don't think that that should prevent anybody," Kalina said.
The Spanish study suggests that taking two different COVID-19 vaccine doses may actually produce more antibodies.
The study, which has yet to be peer-reviewed, looked at 440 people under the age of 60 who received AstraZeneca as their first shot and then Pfizer as their second dose to evaluate how effective the combination was.
The Spanish study reported that the mixed schedule of doses seemed to jolt the immune system into producing many more antibodies. However, this data only accounts for the antibody level two weeks after receiving the second shot. The researchers acknowledge that more long-term data is needed.
Additionally, the Spanish study didn’t include a group of people who received AstraZeneca twice — the study’s control group simply didn’t receive any second shot — so it is lacking comparative data.
While the data from Spain has yet to be peer-reviewed, Tam said the results are promising.
"That is a very good initial, early sign that this combination has a really robust immune-boosting effect," she said.
Despite the new recommendations from NACI, Kalina said Canada likely won't do away with the AstraZeneca vaccine. He explained that the vaccine is necessary for those who may be allergic to the components of the mRNA vaccines.
"I think that there is going to be an ongoing place for a variety of different vaccines, including the AstraZeneca vaccine, going forward," Kalina said.
Canadian researchers announced earlier this month that they are looking at the effect of using different COVID-19 vaccine doses in Canadian adults to determine if the mix-and-match approach yields a strong immune response and how long that response lasts.
The study involves 1,300 adult participants and will be conducted in conjunction with the COVID-19 Immunity Task Force, Vaccine Surveillance Reference Group, Canadian Immunization Research Network and Dalhousie University.
People 18 years of age or older can participate in the study by clicking here.
Tam said on May 12 she believed Canada would be mixing vaccine doses by the summer. She noted in a press release that data from the new Canadian study will further aid health officials in the fight against COVID-19.
"In addition to international data, this Canadian study will help inform Canada's public health recommendations on the potential to use different combinations of vaccines for the first and second dose, as well as different dosing intervals," Tam said.
As of May 22, more than 13 million Canadians have received at least one dose of the Pfizer COVID-19 vaccine, 3.5 million have received Moderna, and 2.1 million have gotten either AstraZeneca or COVISHIELD, according to data from the federal government.
With files from CTV News' Michel Boyer
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