Moderna or Pfizer: Which vaccine is a better booster? Experts weigh in

Until there's some future vaccine designed specifically to target the highly transmissible Omicron variant, adults in Canada are encouraged to get their booster shot as soon as possible — which means either Pfizer or Moderna.

Both mRNA vaccines offer significant protection against breakthrough infections and hospitalizations, but new research from data collected before Omicron hit shows Moderna's vaccine may offer slightly better protection compared to Pfizer.

That may surprise people who had been turning Moderna down, said Kelly Grindrod, a pharmacist and associate professor at the University of Waterloo school of pharmacy.

"They walk away from it because they think it's a lesser vaccine," she told Dr. Brian Goldman, host of CBC podcast The Dose. "But the evidence doesn't say Moderna is a lesser vaccine. Actually, it's quite the opposite." 

A study published in the peer-reviewed Journal of the American Medical Association (JAMA) last week shows adults who received two doses of the Moderna vaccine had a lower risk of hospitalization compared to those who received two doses of the Pfizer-BioNTech vaccine. The research used data from March to August of last year when Delta was the dominant variant.

As booster shots roll out across the country, Moderna may be a better choice for many over the age of 30 because of its slightly longer protection against infection, said Grindrod, the pharmacy vaccine lead for the Waterloo region.

New research around which vaccine is best for specific populations — and Omicron — is evolving, but there are a few key things you need to know right now.

How do the two vaccines compare?

Doctors and public health officials have said that people should get whichever booster shot is readily available to them. 

Both mRNA vaccines offer significant protection against COVID-19, Grindrod said, but studies comparing the two vaccines are showing Moderna has a slight edge. 

"Now, if you're someone who's in your 40s and is otherwise healthy, is that going to be a huge difference for you? Probably not. It might be just a very small difference," she said.

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But there is some research showing that for older populations or for those who have weaker immune systems, Moderna may offer slightly more protection against breakthrough infections, Grindrod said. 

The researchers who published the study in JAMA found that although immune responses to Moderna and Pfizer's vaccines dropped six months after vaccination, antibody titers — a test used to detect the level of antibodies in blood — were higher with Moderna. 

In another study published in the New England Journal of Medicine in December, researchers looked at the effectiveness of the two vaccines among health-care workers between December 2020 and May 2021.  

It found that vaccine effectiveness after two doses were 88.8 per cent for Pfizer and 96.3 per cent for Moderna. 

"Moderna does seem to have a more durable protection, which is going to protect you for potentially a bit longer against those breakthrough infections," Grindrod said.

She added there are still a lot of questions about why Moderna is outperforming Pfizer. 

She said it may be because the dosage is different between the vaccines. The regular full dose of Moderna is 100 micrograms of nucleoside-modified messenger RNA (mRNA) compared to Pfizer's full dose vaccine which is 30 micrograms.

The Moderna booster shot is a half dose. The Pfizer-BioNTech booster is a full dose.

"There's actually more mRNA in the half-dose Moderna than the full dose Pfizer. So when we're looking at protection, this slightly higher dose might actually give a slightly more durable immunity when it's a booster shot," Grindrod said. More data on booster shots is still needed, she said.

Which mRNA vaccine better protects me from Omicron?

New research is showing how well boosters are holding up in the Omicron wave — still protecting against infection, symptoms and hospitalization. 

But how long that protection lasts is still unknown, Grindrod said. 

A report from the U.S. Centers for Disease Control and Prevention found that vaccine effectiveness against Omicron hospitalization fell to just 57 per cent in people who were more than six months out from a second shot, but grew to 90 per cent with a booster.

A second CDC report looked at 25 U.S. states between April and December and similarly found that while defence against infection waned, those with boosters had much higher protection against symptoms both when Delta was dominant and when Omicron overtook it.

And a third study from CDC researchers published in the the Journal of the American Medical Association found three shots of mRNA vaccines were close to 67 per cent effective against symptomatic disease from Omicron compared to the unvaccinated.

It's not clear yet which booster is most effective against Omicron.

Which vaccine should I get for my booster shot?

Health Canada has authorized Moderna's COVID-19 vaccine (also known as Spikevax) and Pfizer-BioNTech's vaccine (Comirnaty) for booster-shot use in people 18 or older. 

Whether or not you will be offered a COVID-19 booster shot is based on each province or territory's eligibility criteria and rollout plan.

Which vaccine you get depends on your age and where you live. In some places, adults may be offered a choice.

In Toronto, Durham and Waterloo regions, people aged 30 and up will only be offered Moderna.

When it comes to 18 to 29-year-olds, Grindrod said Pfizer is generally the recommended vaccine.

Nova Scotia is recommending people between 18 and 24 receive the Pfizer vaccine due to  "a rare, but increased risk of myocarditis/pericarditis in young adults from the Moderna vaccine as compared to the Pfizer vaccine."

Last fall, the Ontario government offered a similar recommendation due to a "mild risk" of the rare heart condition myocarditis "out of an abundance of caution." 

Heart inflammation after a COVID-19 infection can also happen, and doctors and other health experts have emphasized that the benefits of getting a COVID-19 vaccine outweigh the risk of heart inflammation.

Take what you can get, doctors say

If you're 30-years-old or older and healthy, Grindrod said people should just take the first shot they can get. 

And if you received the AstraZeneca or Johnson & Johnson shots for your first two doses, getting either the Moderna or Pfizer boosters that are available will still offer strong protection against COVID-19. 

But if someone has a weakened immune system or has more risk factors, Grindrod leans towards recommending Moderna for a booster shot.

Dr. Iris Gorfinkel, a Toronto-based family doctor and vaccine researcher, emphasized that when it comes to reducing hospitalizations, both are "extremely effective."

"It's not promising to be perfect, even when it comes to hospitalizations. We're going to see breakthrough infections with either vaccines, but those breakthrough infections are few and far between compared to populations who remain unvaccinated."

Gorfinkel emphasizes that the differences between Pfizer and Moderna's vaccines "are small."

"The truth is they both really work well."

Both Pfizer and Moderna have launched studies of Omicron-specific boosters.

Gorfinkel said it will  take real-world study data comparing the vaccines head-to-head against Omicron to really tell if one offers better protection.

Written and produced by Stephanie Dubois, with files from Adam Miller and Amina Zafar.