A vial of AstraZeneca vaccine is seen at a mass COVID-19 vaccination clinic in Calgary on April 22, 2021. THE CANADIAN PRESS/Jeff McIntosh

A vial of AstraZeneca vaccine is seen at a mass COVID-19 vaccination clinic in Calgary on April 22, 2021. THE CANADIAN PRESS/Jeff McIntosh

10 weeks or 12? Expert says slight difference in AZ dosing interval likely negligible

Health Canada says the gap between shots can stretch 4 to 12 weeks

A push to administer thousands of COVID-19 vaccines before they expire has shortened the recommended dosing interval for some Ontarians, but one expert says that any difference in protection is likely negligible.

The province announced last week that the first cohort of Ontarians who received the Oxford-AstraZeneca shot in March were now eligible to schedule second jabs — marking 10 weeks from their first shot.

A recent study out of the United Kingdom found that lengthening AstraZeneca’s dosing interval to 12 weeks or more was more effective against COVID-19 than administering the doses six weeks apart.

The findings seemed to back Canada’s recommended strategy to delay second doses in order to administer more first doses. Health Canada says the gap between shots can stretch 4 to 12 weeks, while the National Advisory Committee on Immunization has recommended an extended interval of up to 16 weeks.

Dr. Allison McGeer, a professor in laboratory medicine and pathobiology at the University of Toronto, says taking a second vaccine dose at 10 weeks instead of 12 likely won’t impact its effectiveness.

“I can tell you that 12 weeks is better than four weeks, but I can’t tell you whether … 12 weeks is better than 10,” McGeer said during an Ontario Medical Association virtual press briefing on Wednesday.

“(But) personally, from my experience with vaccines, I’m with the province on this one.

“I do not think that your (immune) response is going to be different at 10 weeks than it is at 12 weeks.”

While some early AstraZeneca recipients may be wondering if they’re getting shortchanged by getting a second dose earlier, McGeer said vaccine schedules are typically “invented.”

“We decide what we think will work, and then we try it out and if it works we say, ‘Great,’ and we move on,” she said. “We don’t spend a lot of time saying: ‘Well, if we vaccinate you at 10 weeks or 12 weeks or 14 weeks or six weeks, is that a little bit better?’”

Ontario administered nearly one million doses of the AstraZeneca vaccine before halting first shots earlier this month due to what it said was an increased risk of blood clots.

The jab has been associated with vaccine-induced immune thrombotic thrombocytopenia (VITT), an extremely rare blood clotting condition that while treatable, can also be fatal.

The chances of acquiring VITT from a second shot of AstraZeneca are believed to be more rare than with the first dose, though McGeer said an exact estimate of risk is still unknown.

Those who received AstraZeneca for their first dose may be eligible for an mRNA vaccine from Pfizer-BioNTech or Moderna for their second shot, but the National Advisory Committee on Immunization is still waiting for efficacy data from a U.K. trial examining that mixed-dosing strategy.

McGeer said waiting for a potential mRNA dose could also put people at risk of contracting COVID-19 in the meantimebecause they’re not as fully protected from the virus.

“Now you’re balancing the ability to get a second dose of vaccine earlier, versus waiting for your second dose of an mRNA vaccine,” McGeer said.

READ MORE: NACI recommends same vaccine for 2nd dose; more data on mixing in mRNA vaccines coming

Ontario has about 45,000 doses of AstraZeneca expiring Monday,and pharmacies are aiming to dispense them within the next few days to avoid wastage. Another 10,000 doses of AstraZeneca expire next month, and the province has more than 300,000 doses in stock.

Dr. Zain Chagla, an infectious disease expert in Hamilton, says that with a steady, increased supply of mRNA vaccines coming in, it may be time for Canada to re-assess the amount of doses it needs right now.

He says some future shipments could be more useful in other countries that don’t have equitable access to the same inoculations.

“Does that mean we don’t vaccinate our population? No … but we do have to make difficult decisions about what the next three months of our vaccine plan looks like,” Chagla said. “Where do we set the cutoff to say: ‘OK, we’re not going to accept any more AstraZeneca into Canada … where is the cut off for Johnson & Johnson?

“Do we need this as part of our vaccine strategy or can we give that back to the world?”

Melissa Couto Zuber, The Canadian Press


Like us on Facebook and follow us on Twitter.

Coronavirusvaccines

Just Posted

Police officers and their dogs undergo training at the RCMP Police Dog Services training centre in Innisfail, Alta., on Wednesday, July 15, 2015. Mounties say they are searching for an armed and dangerous man near a provincial park in northern Alberta who is believed to have shot and killed a service dog during a police chase. THE CANADIAN PRESS/Jeff McIntosh
RCMP search for armed man in northern Alberta after police dog shot and killed

Cpl. Deanna Fontaine says a police service dog named Jago was shot during the pursuit

Alberta now has 2,336 active cases of COVID-19, with 237 people in hospital, including 58 in intensive care. (Black Press file photo)
Red Deer down to 73 active cases of COVID-19, lowest since early November

The Central zone has 253 active cases of the virus

File photo
Update: Leduc RCMP request assistance to identify armed robbery suspect

Leduc RCMP are searching for suspect involved in an armed robbery at the Leduc Giant Tiger.

People line up to get their COVID-19 vaccine at a vaccination centre, Thursday, June 10, 2021 in Montreal. THE CANADIAN PRESS/Ryan Remiorz
Vaccines, low COVID case counts increase Father’s Day hope, but risk is still there

Expert says people will have to do their own risk calculus before popping in on Papa

Canadian Prime Minister Justin Trudeau is seen during a joint news conference following the EU-Canada Summit, in Brussels, Belgium, Tuesday June 15, 2021. Trudeau says Canada is on track now to have 68 million doses delivered by the end of July, which is more than enough to fully vaccinate all 33.2 million Canadians over the age of 12. THE CANADIAN PRESS/Adrian Wyld
Vaccine deliveries enough to fully vaccinate all eligible Canadians by end of July

Three in four eligible Canadians now have their first dose, nearly one in five fully vaccinated.

Chief Public Health Officer Theresa Tam listens to a question during a news conference, in Ottawa, Tuesday, Jan. 12, 2021. The number of confirmed COVID-19 cases attributed to the highly contagious Delta variant grew in Canada this week. THE CANADIAN PRESS/Adrian Wyld
Canada’s public health agency reports spike in confirmed cases of Delta variant

More than 2,000 cases of the variant confirmed across all 10 provinces and in one territory

The federal government says it wants to ban most flavoured vaping products in a bid to reduce their appeal to youth. THE CANADIAN PRESS/AP-Craig Mitchelldyer
Health Canada proposes ban on most vaping flavours it says appeal to youth

If implemented, the regulations would restrict all e-cigarette flavours except tobacco, mint and menthol

The Montreal Police logo is seen in Montreal on Wednesday, July 8, 2020. Some Quebec politicians are calling for an investigation after a video was released that appears to show a Montreal police officer with his leg on a young Black man’s neck during an arrest. THE CANADIAN PRESS/Paul Chiasson
Probe called for after video appearing to show Montreal officer’s knee on Black youth’s neck

Politicians call for investigation after clip evokes memories of George Floyd incident

Thousands of protesters make their way through the downtown core during a Black Lives Matter protest in Ottawa, Friday June 5, 2020. THE CANADIAN PRESS/Adrian Wyld
MPs’ study of systemic racism in policing concludes RCMP needs new model

Chair of the House public safety committee says it’s time for a reckoning on ‘quasi-military’ structure

A case filled with packages of boneless chicken breasts is shown in a grocery store Sunday, May 10, 2020, in southeast Denver. THE CANADIAN PRESS/AP-David Zalubowski
One million chickens euthanized during labour dispute at Quebec slaughterhouse

Premier says waste amounts to 13 per cent of the province’s chicken production thrown in the garbage

A section of the eastern slopes of the Canadian Rockies is seen west of Cochrane, Alta., Thursday, June 17, 2021. A joint federal-provincial review has denied an application for an open-pit coal mine in Alberta’s Rocky Mountains, saying its impacts on the environment and Indigenous rights aren’t worth the economic benefits it would bring. THE CANADIAN PRESS/Jeff McIntosh
Panel says Grassy Mountain coal mine in Alberta Rockies not in public interest

Public hearings on the project in southern Alberta’s Crowsnest Pass region were held last fall

Most Read