Pfizer is not superior to Moderna and it’s ‘perfectly okay’ to mix COVID vaccines, experts say

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Dr. Fahad Razak was pulling into a Toronto vaccine centre last weekend when a woman, clearly frustrated about something, gestured for him to roll down his window. “They don’t have Pfizer,” the exasperated woman said. “They only have Moderna.”

Razak was puzzled by his encounter with the perfect stranger. “I’m surprised, and intrigued and curious about why there has been such a popular imagination, a fixation, on Pfizer as being superior to Moderna, when there is no evidence that I, or any scientist that I work with — and we’ve looked extensively — can find to support even the smallest claim that one is better than the other.”

While health authorities are reassuring Canadians that it’s safe to mix COVID-19 vaccines doses, and that doing so may even produce a more robust immune response, some appear to be rejecting Moderna for Pfizer.

Ontario pharmacies are reporting cancellations, walkouts, no-shows and lack of interest for Moderna, after Ontario and other provinces scaled up Moderna doses this week because of stalled Pfizer shipments. Not every pharmacy is struggling, but “we know that there is supply sitting still in pharmacies for Moderna,” Justin Bates, CEO of the Ontario Pharmacists Association said Wednesday.

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Ontario has now received this week’s delayed Pfizer doses, and allocations are being sent to public health units, but with more Pfizer shipments expected to lag Moderna in coming weeks, Bates and others are calling for stronger public health messaging to clear up confusion about vaccine “interchangeability,” preferred brands and whether there is any downside to getting a second dose sooner than expected.

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Razak, an internal medicine specialist at St. Michael’s Hospital in Toronto, is hopeful that only a small minority of the public views Pfizer as superior to Moderna. “There may be some brand recognition and recall happening,” he said. “I don’t know if that has influenced people’s perception.” His wife received AstraZeneca for her first dose, and, this past weekend, Moderna for her second. “I certainly would not be saying this publicly and doing something different for my family,” Razak said.

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The no-shows and walkaways for Moderna don’t appear to be having a wide impact, infectious diseases physician Dr. Isaac Bogoch, a member of the province’s COVID-19 vaccine distribution task force, tweeted Tuesday. More than 207,800 COVID shots were administered in Ontario on Tuesday alone; most (88 per cent) were second doses.

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But the brand shopping has health authorities across the country concerned about any reluctance to swap. The formulas may not be identical, but they are “extremely similar and it’s perfectly okay to get one dose of each,” Dr. Deena Hinshaw, Alberta’s chief medical officer of health, said in a series of tweets Monday. “We will do our best to make sure that we have both products available at our clinics, but sometimes that simply isn’t possible,” Dr. Bonnie Henry, British Columbia’s provincial health officer said.

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It’s also crucial that anyone considering only one dose of any vaccine knows that one dose doesn’t quite cut it.

Early results from a Canadian study based on nearly 6,000 dried blood spot samples collected from Feb. 8 to May 17 show a “high degree of variability” in the level of antibodies produced by just a single shot of a COVID vaccine.

Approximately 10 per cent of people who reported being vaccinated with a single dose of an mRNA vaccine (Pfizer-BioNTech or Moderna), and 30 per cent of those vaccinated with Oxford-AstraZeneca did not show signs of antibody levels “above thresholds differentiating them from the population at large,” meaning the unvaccinated, Dr. Philip Awadala, national scientific director of CanPath, which led the study, said in a statement.

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A single dose of Pfizer or Moderna produced short-term antibody levels more than one-and-a-half times greater than those produced by AstraZeneca. Antibody levels in people with two doses of Pfizer or Moderna were almost twice as high as those after the first dose.

The early results don’t include people who received two doses of AstraZeneca.

In Quebec, people still need to wait eight weeks at least between doses, regardless of the vaccine.

In Ontario, people must wait a minimum of eight weeks after a first dose of AstraZeneca, but only a minimum of 28 days after a first dose of Pfizer or Moderna.

Pfizer and Moderna were formally authorized for use 21 and 28 days apart, respectively. But the choice of three weeks or four “wasn’t based on decades of vaccinology,” said McMaster University immunologist Dawn Bowdish. “It was based on, what’s the minimal amount of time you can squish doses together to get everyone protected?”

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“Pilot data coming out says, yes, actually, if you wait a little longer between doses you get a little bit of a bump in your immune response,” Bowdish said. In March, Canada’s vaccine advisors said doses could be spaced up to four months apart. With growing vaccine supplies, and the Delta variant spreading, the advisory panel now says second doses should be given as soon as possible.

The risk with Delta is that it can sneak under, and partially dodge the immune response in people dosed only once. As of June 22, 67 per cent of Canada’s population had received one dose of a COVID vaccine, but just 20.7 per cent had two. The gap, Razak said, needs to be narrowed

“We need to go hard and fast” on vaccinations, agreed Bowdish, who is a Moderna times two recipient, and who would have been just as happy to have had a Moderna/Pfizer mix. “And I would remind people that every flu shot you’ve ever got, you’ve never known the manufacturer.” Booster shots are highly likely in the future, “and I suspect in the long term we will not care so much. Whatever one your local pharmacy is offering is the one you’ll probably get.”

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Pfizer or Moderna are now considered the preferred second dose for people who received a first dose of AstraZeneca. In some provinces, people first vaccinated with AZ can choose whether they want to opt for an mRNA or get a second dose of AstraZeneca. In other provinces, they’re just being offered mRNA, simply because there’s no more AZ around.

But Pfizer and Moderna are virtually identical “in terms of what they’re doing, and how they do it,” said Dr. Catherine Hankins, co-chair of Canada’s COVID-19 immunity task force. COVID vaccines are pulling us out of the pandemic and “there’s no point in shopping around between Pfizer and Moderna.”

Each shows the immune system a slightly different part of the virus by using slightly different sequences of the spike protein — the key that allows the virus to enter human cells, and the key target of the immune system.

With Pfizer followed by Moderna, or vice versa, “you get the best of both worlds,” Bowdish said. “When the real virus gets to you, you’ve got broader protection.”

A preprint study by researchers in Germany suggests AZ combined with Pfizer produces higher concentrations of neutralizing antibodies compared to two doses of AstraZeneca.

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