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

Joseph Albert Brooks, 94-years-young pf Prince Rupert offers traditional prayers and smudging to the sick. (Photo: K-J Millar/The Northern View)
Heart of our City: Joseph Albert Brooks keeps smudging and praying for others

94-year-old Tsimshian elder just wants some help washing his floors

Land along Prince Rupert’s waterfront, PID 012-247-391, where residents say excessive industrial train noise is stemming from, has been found to be owned by the City of Prince Rupert and is not federal land like first presented, Prince Rupert Environmental Society stated on June 17. (Image: supplied by Land Title and Survey, Govt. of BC.)
Error found on land titles map may assist city with noise control enforcement of industry

Prince Rupert residents had been told there was no municipal jurisdiction to enforce noise bylaws

Department of Oceans and Fisheries has announced as of July 19 chinook salmon is not to be fished in certain areas in BC tidal waters until July. Spring chinook salmon are seen swimming. (Photo courtesy Michael Humling, US Fish & Wildlife Service)
Chinook Salmon limits set to zero in some BC tidal waters

DFO implement restrictions to protect Chinook Salmon

Visitors to a pop-up temporary aquarium in Prince Rupert will have the chance to see marine ecology from July 21 to Aug. 15, like this viewer watching sea anemones at the Ripley’s Aquarium in Toronto. (Photo: K-J Millar/The Northern View)
Prince Rupert pop-up aquarium will bring sea level to eye level in July

A permanent peak to reef ecology centre is in the planning stages by North Coast Ecology Society

Prince Rupert’s Ellen Wright and Graeme Dickens jam out during filming the two Ring System Studio concerts to be broadcast on television during June. (Photo: supplied, H. Cox)
Ring System Studio sounds on television

Two concerts by the Prince Rupert music school will be broadcast in June

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

A North Vancouver man was arrested Friday and three police officers were injured after a 10-person broke out at English Bay. (Youtube/Screen grab)
Man arrested, 3 police injured during 10-person brawl at Vancouver beach

The arrest was captured on video by bystanders, many of whom heckled the officers as they struggled with the handcuffed man

Patrick O’Brien, a 75-year-old fisherman, went missing near Port Angeles Thursday evening. (Courtesy of U.S. Coast Guard)
Search for lost fisherman near Victoria suspended, U.S. Coast Guard says

The 75-year-old man was reported missing Thursday evening

Bruce Springsteen performs at the 13th annual Stand Up For Heroes benefit concert in support of the Bob Woodruff Foundation in New York on Nov. 4, 2019. (Greg Allen/Invision/AP)
Canadians who got AstraZeneca shot can now see ‘Springsteen on Broadway’

B.C. mayor David Screech who received his second AstraZeneca dose last week can now attend the show

New research suggests wolves can be steered away from the endangered caribou herds they prey on by making the man-made trails they use to hunt harder to move along. THE CANADIAN PRESS/Nathan Denette
Culling cutlines, not B.C. wolves, key to preserving caribou herds: researcher

The government has turned to killing hundreds of wolves in an effort to keep caribou around

Gary Abbott (left) and Louis De Jaeger were two of the organizers for the 2014 Spirit of the People Powwow in Chilliwack. Monday, June 21, 2021 is Indigenous Peoples Day. (Jenna Hauck/ Chilliwack Progress file)
Unofficial holidays: Here’s what people are celebrating for the week of June 20 to 26

Indigenous Peoples Day, Take Your Dog to Work Day, Onion Rings Day all coming up this week

Gwen Spencer Hethey with her uncle and mentor Major Frederick Richardson. (Courtesy of Greater Victoria Sports Hall of Fame)
‘She was a killer’: The B.C. woman who pioneered female sharpshooting

Gwen Spencer Hethey made military men ‘look like turkeys’ says her son

Central Okanagan Grade 12 grads are set to get $500 each after a more than $1 million donation from a Kelowna couple. (File photo)
B.C. couple donating $500 to every Grade 12 student in the Okanagan

Anonymous donors identified as Kelowna entrepreneurs Lance and Tammy Torgerson

Most Read