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

Two volunteers work to sieve a sample of sand and ocean water through a filter, capturing any potential microplastics. (Courtesy of Ocean Diagnostics)
Victoria startup making waves in microplastics research

New products from Ocean Diagnostics will make research faster, more affordable

Willows Beach in Oak Bay. (Black Press Media file photo)
Seven days of sun set to shine on Greater Victoria

Special weather statement warns of higher than usual temperatures

Chef Trevor Randle leads a June 21 online cooking featuring recipes – beef zesty lettuce wraps, blueberry strudel and blueberry spritzer. (Courtesy We Heart Local BC)
Free online cooking course explores B.C. blueberries and beef

Chef Trevor Randle calls them the province’s most flavourful foods

Google Maps shows significant traffic backups after a crash reported shortly before noon on Father’s Day, June 20. (Google Maps)
Father’s Day crash in Saanich closes lane of McKenzie Avenue

Police say there were injuries, traffic impacted

Victoria Regional Transit System routes 7 UVic/Downtown, 21 Interurban/Downtown and 53 Colwood/Langford will undergo changes as of June 28. (Black Press Media file photo)
Greater Victoria school routes scaled down for summer by BC Transit

Service changes take effect June 28, enhanced service returns this fall and winter

Jesse Roper tackles weeds in his garden to kick off the 2021 season of What’s In My Garden Man? (YouTube/Whats In My Garden)
VIDEO: Metchosin singer-songwriter Jesse Roper invites gardeners into his plot

What’s In My Garden, Man? kicks off with the poop on compost

Blair Lebsack, owner of RGE RD restaurant, poses for a portrait in the dining room, in Edmonton, Friday, June 18, 2021. Canadian restaurants are having to find ways to deal with the rising cost of food. THE CANADIAN PRESS/Jason Franson
Canadian restaurateurs grapple with rising food costs, menu prices expected to rise

Restaurants are a low margin industry, so there’s not a lot of room to work in additional costs

Cpl. Scott MacLeod and Police Service Dog Jago. Jago was killed in the line of duty on Thursday, June 17. (RCMP)
Abbotsford police, RCMP grieve 4-year-old service dog killed in line of duty

Jago killed by armed suspect during ‘high-risk’ incident in Alberta

A Lotto 6/49 ticket purchased in Parksville for the June 19, 2021 draw is a $3M winner. (Submitted photo)
Winning Lotto 6/49 ticket worth $3M purchased on Vancouver Island

Lottery prize winners have 52 weeks to claim jackpot

The George Road wildfire near Lytton, B.C., has grown to 250 hectares. (BC Wildfire Service)
B.C. drone sighting halts helicopters fighting 250 hectares of wildfire

‘If a drone collides with firefighting aircraft the consequences could be deadly,’ says BC Wildfire Service

Barbara Violo, pharmacist and owner of The Junction Chemist Pharmacy, draws up a dose behind vials of both Pfizer-BioNTech and Oxford-AstraZeneca COVID-19 vaccines on the counter, in Toronto, Friday, June 18, 2021. An independent vaccine tracker website founded by a University of Saskatchewan student says just over 20 per cent of eligible Canadians — those 12 years old and above — are now fully vaccinated. THE CANADIAN PRESS/Nathan Denette
At least 20% of eligible Canadians fully vaccinated, 75% with one dose: data

Earlier projections for reopening at this milestone didn’t include Delta variant

RCMP crest. (Black Press Media files)
Fort St. John man arrested after allegedly inviting sexual touching from children

Two children reported the incident to a trusted adult right away

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

This undated file photo provided by Ernie Carswell & Partners shows the home featured in the opening and closing scenes of The Brady Bunch in Los Angeles. Do you know the occupation of Mike Brady, the father in this show about a blended family? (Anthony Barcelo/Ernie Carswell & Partners via AP, File)
QUIZ: A celebration of dad on Father’s Day

How much do you know about famous fathers?

Most Read