TORONTO -- Experts are reminding Canadians to get the first vaccine that is available to them as some, particularly those who received AstraZeneca as their first shot, turn down a Moderna jab in favour of its Pfizer-BioNTech counterpart.

Dr. Dale Kalina, an infectious disease physician at Joseph Brant Hospital in Burlington, Ont., told that Moderna and Pfizer are "very similar" vaccines and Canadians shouldn’t be hesitant to get one over the other.

"I think there is always a push for people to be competitive about getting the best of a vaccine, or a car or whatever it may be and that's just something that's part of our society," Kalina said in a telephone interview on Tuesday.

"But it's important to recognize that the Moderna and Pfizer vaccines… are being accounted as interchangeable for the populations for whom it's been approved," he added.

Canada's National Advisory Committee on Immunization (NACI) changed its guidelines on June 1 to allow for mixing and matching COVID-19 vaccines, saying that a first dose of the AstraZeneca-Oxford/COVISHIELD vaccine can be followed up with a second AstraZeneca shot, or be safely combined with a second dose of the Pfizer-BioNTech or Moderna shots.

The new guidance from NACI also advises that the Pfizer and Moderna vaccines can be mixed for first and second doses.

"NACI has worked to quickly adapt this guidance on the use of COVID-19 vaccines in Canada to ensure optimal protection of Canadians at pace with the ever changing circumstances during this pandemic," Canada's Chief Public Health Officer Dr. Theresa Tam said during the briefing on the updated guidance.

"This advice provides provinces and territories with safe and effective options to manage the vaccine programs," she added.

If Canadians who received AstraZeneca as their first dose are open to receiving Moderna as their second shot, infectious disease expert Dr. Abdu Sharkawy says it "will be much easier to accommodate for everyone."

Sharkawy noted in a series of recent tweets that there is nothing to compare between Pfizer and Moderna except for the brand names.

"…Only an inorganic chemist could be excited about the differences between Pfizer/Moderna. SAME mechanism, efficacy vs variants, overall safety profile. Wanna get excited? Get vaccinated. Get closer to safety & freedom. Faster," Sharkawy tweeted, in part.

According to data from clinical trials, Moderna showed 94.1 per cent efficacy in preventing COVID-19 two weeks after a second dose, comparable to Pfizer's 95 per cent. Both trials saw zero hospitalizations and deaths in those who contracted COVID-19 after being fully vaccinated.

Side effects from each of the mRNA vaccines are also similar, with neither producing more adverse events than the other.

Despite this, health experts and pharmacists in Ontario and Alberta have reported people walking out of clinics once they realized they'd be getting the Moderna jab.

Sharkawy said in another tweet that "Moderna is as good as Pfizer," and refusing the first for the latter would be comparable to declining a free meal because it was delivered via DoorDash instead of SkipTheDishes.

"Sound ridiculous? Of course it does. Stop being #NoneThePfizer You gotta learna to get on #Moderna," Sharkawy tweeted.

Kalina said some of the favouritism towards getting Pfizer over Moderna comes from the current data. He said studies about mixing COVID-19 vaccines have primarily focused on combining shots of AstraZeneca and Pfizer. Kalina said this is because these studies have been completed in the U.K. where Moderna hasn’t been authorized for use.

"But the reality is the Moderna vaccine and the Pfizer vaccine employ the same technology and are very, very similar vaccines. So I would expect that [Moderna] would actually have the same response to mixing as the Pfizer vaccine with AstraZeneca," he said.

While there has been public confusion about some of the messaging on COVID-19 vaccines, Kalina noted that the scientists and physicians who work at NACI "know what they're talking about."

"We've seen NACI making recommendations… for about eight to 10 months now, and the recommendations have been based on not only what we know about the current COVID vaccines, but also what we know about vaccines in general," he explained.

"They've said that Pfizer and Moderna are equally effective, and that should be taken at face value," he added.

Amid previous issues with unsteady supply from the company, Canada is set to receive a total of 7.1 million Moderna doses in two separate shipments this week.

The two shipments will more than double the number of shots Moderna has sent to Canada since the vaccine was approved in December 2020. As of last week, the Massachusetts-based pharmaceutical firm had only delivered 6.2 million doses, according to the Public Health Agency of Canada.

With the country expecting an influx of Moderna shots, Kalina said it is important to remind Canadians who are mixing vaccines that a second dose of either Moderna or Pfizer are equivalent in efficacy.

"The Moderna and the Pfizer vaccines are both exceptionally good, and the most important thing is that you get your first and your second doses as quick as you can," Kalina said.

With files from The Canadian Press