OTTAWA -- Many immunologists and infectious disease experts are rushing to convince Canadians that extending the expiration date on some Oxford-AstraZeneca doses is a normal and scientifically sound decision.

Health Canada said Saturday it had approved adding another month to the shelf life of two lots of Oxford-AstraZeneca vaccine that were due to expire Monday.

Ontario asked the company if the change could happen to save up to 45,000 doses that were expiring, but the decision affects remaining doses from the two affected lots in every province. Fewer than 100,000 doses are left from those two lots.

Health Canada's chief medical adviser, Dr. Supriya Sharma, said in an interview with The Canadian Press Monday that the decision was made after AstraZeneca submitted data supporting the change last week.

That included the results of testing doses from the two lots that were expiring, and modelling data from the company that showed the doses would remain stable for at least the next month.

"We've been really clear that we're making decisions based only on science and evidence, and this is no exception," Sharma said.

But the decision to adjust the expiration date at the 11th hour on the weekend led many Canadians to express frustration and concern about whether it was truly being made because of science, or because of a policy failure to use the doses in time.

"The feedback I'm getting is this idea that it was done out of desperation," said Timothy Caulfield, the Canada Research Chair in health law and policy at the University of Alberta.

"That's the way it looks from the outside. It's a really good example of why context matters."

Pharmacologist Sabina Vohra-Miller, founder of the science communication website Ambiguous Science, said it is not uncommon at all for companies to get calls from pharmacies asking if a patient can use their drug beyond the expiration.

"Most of the time when these expiry dates are extended, people don't get to know about it," she said.

She said vaccines and other medications are tested constantly to determine how they perform in various scenarios, including past their expiration.

"For all we know we might next year find out that based on long-term shelf life studies that in fact the vaccines are totally OK for an entire year, so maybe for future batches the expiry date will be a year," she said. "We just don't know this yet, because these vaccines are so new."

AstraZeneca, Pfizer-BioNTech and Moderna all have six-month life spans at the moment, if stored properly. That means in a fridge for AstraZeneca, an ultralow temperature freezer for Pfizer and a normal freezer for Moderna.

The Johnson & Johnson vaccine, the only other one authorized by Health Canada though it's not in use here yet, can be stored frozen for up to two years.

Sharma said there are no other requests to adjust expirations for COVID-19 vaccines, but also noted that doses expiring is not a big issue when people still want more doses than we have in our supply.

It was an issue for these doses because Ontario had paused the use of AstraZeneca while awaiting more data on the risk of vaccine-induced blood clots. It is now using them only for second doses but didn't decide to proceed with second doses early enough to avoid the risk the expiring doses would go to waste.

Sharma said she can understand the perception that the decision was made out of desperation but said that is not something Health Canada would do.

"I mean, I think it would have been better if the submission came to us earlier," she said. "It only came to us on May 27. And so we looked at it as soon as we could. If that data did not support the extension, we would have said no."

Caulfield said communicating scientific decisions in a pandemic is really hard, but doing it well is also critical.

"When I first heard this story my immediate reaction was anything that invites doubt, anything that injects more skepticism into the calculus, for those that are sitting on the fence (about getting vaccinated) is problematic," he said.

More than two-thirds of eligible Canadians are now vaccinated with at least one dose. Caulfield said Canada is getting close to what he calls the "hesitancy hurdle" where the people not yet vaccinated may need some convincing.

The impact of framing how we talk about vaccines matters "more and more" right now, he said.

Dr. Allison McGeer, an infectious disease consultant at Mount Sinai Hospital in Toronto, said the change to the vaccine expiry is a normal part of the process and should not be viewed with alarm.

"Yeah, it will come at the last minute but that's a pandemic thing," she said. "I have absolute faith in Health Canada, that they know what they're doing, and that we have solid processes for managing this and extending expiry dates. This isn't anything out of the ordinary."

With files from Melissa Couto in Toronto.

This report by The Canadian Press was first published May 31, 2021.