Ontario stands firm on 12-week interval for AstraZeneca doses — against experts' advice

Citing the need for speed against the growing threat of the coronavirus delta variant, infectious diseases specialists say the wait for a second dose should only be eight weeks, regardless of what kind of vaccine people got as their first dose.

Gap between vaccine doses is only 8 weeks in other provinces

People in Ontario who got the AstraZeneca vaccine as their first dose must wait 12 weeks to get a second dose, even if they choose to get an mRNA vaccine the second time around. People who got an mRNA vaccine as their first shot only need to wait eight weeks, according to provincial government rules. (Sam Nar/CBC)

Citing the need for speed against the growing threat of the coronavirus delta variant, infectious diseases specialists say they disagree with the Ontario government's decision to maintain a 12-week waiting period before people who received a first dose of the AstraZeneca-Oxford vaccine can get their second dose.

That rule applies even for people who got a first shot of AstraZeneca and are choosing an mRNA vaccine (Pfizer-BioNTech or Moderna) for their second. 

In contrast, people in Ontario who received an mRNA vaccine as their first dose can get their second one after just eight weeks. 

Other provinces, including B.C., Alberta and Quebec, are now recommending that everyone get their second dose after eight weeks, regardless of whether the first jab was AstraZeneca or an mRNA vaccine. 

Speed vital to counter delta variant

Infectious disease experts widely agree that the two doses required to be fully vaccinated against coronavirus will offer stronger protection against the delta variant of the virus, which has become the dominant strain in the U.K. and is gaining momentum in Canada. 

"The push should really be to get those second doses in fast," said Dr. Isaac Bogoch, an infectious disease specialist at Toronto General Hospital, who is also a member of Ontario's COVID-19 Vaccine Distribution Task Force. 

"Treat a crisis like a crisis," Bogoch said. "Even though case numbers are declining and hospitals are decompressing and vaccines are rolling out — and clearly that's all good — you still have a delta variant. You still are watching this encroach on the existing variants here.

"You also have a crystal ball. You can look at the United Kingdom and see exactly what's happening there," he said. "This variant will find unvaccinated people and it will find undervaccinated communities. It's doing that in the U.K. and it will do the exact same thing here with us."

Getting not only first doses but also second doses into arms as quickly as possible after the four-week mark (which experts agree is the minimum interval for effectiveness) is vital to stopping that from happening, Bogoch said. 

Decision driven by data, health ministry says

But in an email to CBC News on Thursday, Ontario's ministry of health said its decision is driven by scientific data.   

"We know that two doses of AstraZeneca at a 12-week interval provides a better immune response than over a shorter interval," the ministry said. 

The 12-week interval for people who got the AstraZeneca shot will still apply in delta variant "hotspots," the ministry confirmed to reporters later — even as it starts a program to accelerate second shots for people who got mRNA vaccines in those areas. 

Dr. Dirk Huyer, Ontario's chief coroner and another member of the province's vaccine task force, also defended the 12-week duration between doses, noting that the move is based "upon the data that's available."

There isn't much information yet, Huyer said, about the optimal timeframe between a first dose of AstraZeneca and a second dose of either Pfizer-BioNTech or Moderna. 

"We will continue to look at everything that's available to us," he said. 

Later on Thursday, Dr. Barbara Yaffe, Ontario's associate chief medical officer of health, told reporters that officials would continue to look for more guidance on how to space out mixed doses for people who got a first shot of AstraZeneca but want their second shot to be an mRNA vaccine. 

12-week interval not needed, experts say     

It's true that randomized control trials have shown two doses of AstraZeneca generate the optimal immune response if they are 12 weeks apart, said Dr. Caroline Quach-Thanh, pediatric infectious disease specialist and medical microbiologist at Chu Ste. Justine in Montreal.

But there's no evidence that's the case when people choose an mRNA vaccine instead of AstraZeneca as their second dose, she said. 

"You don't need to wait 12 weeks," said Quach-Thanh, who is the former chair of the National Advisory Committee on Immunization (NACI), but was not speaking on NACI's behalf. 

WATCH | Infectious disease expert says Ontario making wrong call on 12-week dose spacing:

Ontario accelerates 2nd vaccine doses, but not for AstraZeneca

2 years ago
Duration 3:19
Dr. Anna Banerji, a pediatric infectious diseases specialist, explains why she believes the Ontario government is making the wrong decision when it comes to maintaining the 12-week gap between first and second doses of the AstraZeneca-Oxford COVID-19 vaccine.

"I think what comes into play here is to balance," she said. "What needs to be discussed at the Ontario level is this gain in earlier protection for more people when they get their second dose."

"What we all want is to be fully vaccinated before this delta variant starts to get transmitted very actively in the community." 

Dr. Anna Banerji, an infectious diseases specialist at the University of Toronto's Dalla Lana School of Public Health, said that even if someone is taking AstraZeneca for both their doses, the urgency of protecting people against the delta variant is the more important consideration at this point in the pandemic.   

"The risk of exposure is highest now," Banerji told CBC News Network on Thursday. "There's no reason to wait 12 weeks."


  • An earlier version of this story incorrectly said that Dr. Anna Banerji is an infectious disease specialist at St. Michael's Hospital. In fact, Banerji is an infectious disease specialist at the University of Toronto's Dalla Lana School of Public Health.
    Jun 11, 2021 12:17 PM ET


Nicole Ireland is a reporter with The Canadian Press.

With files from CBC News Network

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