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

TEASER PHOTO ONLY
‘Stay-at-home mom’ works to raise $25K to help Options build housing in Surrey

Tammy Bourelle boosts ‘Women of Options’ fundraising campaign, which ends June 30

Surrey Libraries will host a series of free contests and programs for teenagers this summer. (Malin Jordan photo)
Surrey Libraries offering series of free summer programs for teens

Writing contests, virtual programs scheduled throughout summer

Emergency crews responded to a large truck fire under the Alex Fraser Bridge around 8:20 p.m. on Thursday, June 17, 2021. (Christine Tobias/Twitter photo)
Dog dies in truck fire under Alex Fraser Bridge

No people were injured in blaze that consumed two semi-trucks around 8:20 p.m. on Thursday, June 17

Four Surrey students (from left) Charlize Abban from Guildford Park, Mekdim Dereje from North Surrey, Chelsea Michalik from Guildford Park and Sarah Sohail from L.A. Matheson, are some of the latest recipients of $40,000 Cmolik Foundation (Photos: surreyschools.ca)
Four Surrey students receive $40K Cmolik scholarships

Students are from Guildford Park, L.A. Matheson and North Surrey

Surrey City Hall. (File photo)
Surrey ranks eighth in Top-10 list of cities of the future in the Americas

This is according to the Financial Times’ fDi Magazine, a bi-monthly publication out of London, England with a circulation of 15,488

A small pod of Pacific white-sided dolphins pass by close to shore in Campbell River June 16, 2021. Still capture from video courtesy of Kimberly Hart
VIDEO: Dolphin sunset captured from Vancouver Island shore

Spectacular setting for view of travelling pod of Pacific white-sided dolphins

Seth Rogan’s vibrant orange sculpture was sold for $7,000 above Vancouver Art Gallery’s initial estimation at auction Tuesday. June 15. (Heffel Fine Arts)
Vase made by Seth Rogen sells for $12,000 at Vancouver auction

The B.C.-born comedian has a new pot habit, and it’s one that’s paying off

BC Lions running back John White IV (3) runs with the ball during first quarter CFL football action against the Ottawa Redblacks in Ottawa on Saturday, September 21, 2019. THE CANADIAN PRESS/ Patrick Doyle
BC Lions file trademark for new logo

Canadian Football League team files for new design on June 1

The remains of the Mid-Island Co-op in Whiskey Creek along the Alberni Highway on Friday, June 18, after a blaze the day before devastated the gas station. (Michael Briones photo)
VIDEO: Camper van explosion burns Vancouver Island gas station to the ground

Nine fire departments responded to the incident, no injuries reported

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

A vial of AstraZeneca vaccine is seen at a mass COVID-19 vaccination clinic in Calgary, Alta., Thursday, April 22, 2021. The website for a Broadway theatre showing "Springsteen on Broadway" said it would only allow guests "fully vaccinated with an FDA-approved vaccine" — Pfizer-BioNTech, Moderna or Johnson & Johnson. THE CANADIAN PRESS/Jeff McIntosh
No Springsteen for you: AstraZeneca not good enough to qualify for Broadway ticket

Victoria area mayor among those unable to attend New York entertainment due to COVID-19 restriction

Missing Abbotsford man Adam Hobbs was found deceased on Thursday evening (June 17).
Body of missing Abbotsford man Adam Hobbs found

Hobbs was reported missing Monday after leaving his job site in Langley

Audrey Inouye (right) with her two sons Kiyoshi McMillan (left) and Kaito McMillian drop off her milk to hospital volunteer Sylvia Hendel at a human milk depot on October 11, 2012. THE CANADIAN PRESS/Jason Franson
Fraser Health appeals for breast milk after unexplained drop in donations

Give your breast milk and save newborn lives, the health authority is encouraging mothers

The BC Ferries’ website is down for the second time in one week from what they say is likely an overwhelming increase in web traffic. (Black Press Media file photo)
Surging web traffic crashes BC Ferries’ site again

Website down for second time this week

Most Read