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A pharmacist prepares the Pfizer-BioNTech COVID-19 mRNA vaccine at a clinic in Halifax in March.Andrew Vaughan/The Canadian Press

People who received a first shot of the Oxford-AstraZeneca COVID-19 vaccine can receive a second shot from either Pfizer or Moderna, the federal vaccine advisory panel said on Tuesday.

In what it described as a discretionary recommendation, the National Advisory Committee on Immunization, or NACI, also gave the same recommendation for a vaccine made in India, called Covishield, which is comparable to AstraZeneca.

Viral vector vaccines, like those from AstraZeneca, Covishield and Johnson & Johnson, are linked to a blood-clot condition that mRNA vaccines, from Pfizer and Moderna, are not connected to. The risk of that rare side effect is one of several factors leading to the advice that vaccines can be mixed.

The changing advice is also being made because Canada has the luxury of relying on mRNA shots for its mass vaccination campaign and because emerging data show switching vaccines partway through will still lead to a “robust immune response.”

The advisory group noted that mixing vaccine products is already common for many other diseases, including for hepatitis A, influenza, measles, mumps and rubella.

However, NACI doesn’t recommend mixing doses if an mRNA vaccine was administered for the first shot, unless necessary. The advisory panel said Tuesday that an alternate mRNA vaccine should only be used for a second dose when the original vaccine is not readily available.

Provinces and territories make the final decision on how to use the vaccines. On Tuesday, Ontario, Quebec, Saskatchewan, New Brunswick, Prince Edward Island and Alberta all said they would follow NACI’s advice on AstraZeneca vaccine mixing.

“While AstraZeneca is still a good choice, we will offer Albertans a choice,” Alberta Health Minister Tyler Shandro said.

Nova Scotia said it would follow NACI’s advice on AstraZeneca, but the province strayed from its guidance on the mRNA vaccines, and said people who received Pfizer or Moderna as a first dose will be able to choose which mRNA vaccine they want for their second.

Alberta said it would give a choice for people who got a first shot of Moderna.

Manitoba said it recommends everyone who received a first shot of AstraZeneca receive an mRNA vaccine for their second dose. British Columbia said choice will be part of its pending AstraZeneca plan, slated for release on Thursday. And Newfoundland and Labrador said it’s still reviewing the new advice from NACI.

The independent vaccine advisory panel noted in its latest report that countries including Denmark, Finland, France and Germany already recommend an mRNA vaccine be used for a second shot following a first dose of AstraZeneca. The group said that while most countries don’t actively recommend mixing vaccine types, all countries consider people who received two doses of authorized vaccines fully immunized.

NACI relied on a Spanish study for immune-response data. While the results from Spain do not translate directly into a measure of how effective the mix-and-match regimen may be against COVID-19, or how it compares with two doses of Pfizer, the results show that those who received the mix are no less protected, and possibly more protected than they would be with two doses of AstraZeneca.

“The data confirm that the patients in our study have had a very significant boost in their immunity against COVID-19 without major problems,” said Jesus Antonio Frias, head of clinical pharmacology at the La Paz University Hospital in Madrid and a principal investigator on the study.

Dr. Frias said that he and his colleagues measured a greater than sevenfold increase in antibodies that directly block infection in a group of 441 participants who received the Pfizer shot as their second dose. This is greater than would be expected from two doses of AstraZeneca based on earlier clinical studies.

The Spanish researchers also said they did not notice a significant difference in side effects in those who received the mixed doses compared with participants in other studies who received two doses of the same vaccine.

The study, which has not yet been peer reviewed, did not include a parallel group that received two doses of AstraZeneca. While the lack of a direct comparison is “less than ideal,” the overall result showing a strong immunity boost from a mix of doses “is really good news for the world,” said Zhou Xing, a professor of immunology at McMaster University in Hamilton.

Provinces stopped using AstraZeneca for first shots in mid-May amid growing concerns of the rare but serious blood-clot side effect linked to the vaccine and as supply for the vaccine became more uncertain.

The blood-clot syndrome is called vaccine-induced immune thrombotic thrombocytopenia, or VITT. The rate of the syndrome, while still rare, has increased as more data are released, with the Public Health Agency of Canada saying the VITT rate may be as high as 1 in 55,000 doses administered. Internationally, the reported rate has varied between 1 in 26,000 and 1 in 100,000 people vaccinated with a first dose of AstraZeneca.

Following second doses, though, the rate of the blood clots reported in Britain dropped significantly, to about 1 in 600,000, NACI said. While that rate could change as more data is reported, Chief Public Health Officer Dr. Theresa Tam said AstraZeneca was kept as a second-dose option in part because some people want to follow the manufacturer’s recommendation of matching first and second shots.

The blood-clot incidence rate on first doses from Britain has been lower than the data reported by other countries. Andrew Morris, an infectious-diseases physician at Toronto’s Sinai Health and a member of Ontario’s science table, said there is “tremendous uncertainty” around the British data on the VITT rate after second doses. Given the outstanding safety questions and the supply of other vaccines, Dr. Morris said Canada should stop using AstraZeneca.

“In my mind there’s no question that the Pfizer and Moderna vaccines are safer than AstraZeneca,” Dr. Morris said. “From a regulatory point of view, I don’t understand why we’re offering to Canadians an option that is less safe.”

With a report from Carrie Tait in Calgary and Laura Stone in Toronto

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