Infectious disease physician and expert says “there’s no qualitative difference between Pfizer and Moderna” and encourages people to get whichever they can get first. Photo: Roger Starnes Sr/Unsplash
Infectious disease physician and expert says “there’s no qualitative difference between Pfizer and Moderna” and encourages people to get whichever they can get first. Photo: Roger Starnes Sr/Unsplash

One of the lead investigators of a new Canadian research study looking at mixing and matching of COVID-19 vaccines urges people to take the vaccine that’s most readily available rather than shop for a preferred brand.

“There isn’t a strong rationale to do that (vaccine shop). You could use public health measures to try and keep yourself safe until you get that second dose (of a preferred vaccine), but there’s no qualitative difference between Pfizer and Moderna,” Dr. Joanne Langley said in an interview.

“They’re both amazing vaccines.”

Dr. Joanne Langley

Langley is a professor of pediatrics and community health and epidemiology at Dalhousie University and is based at the IWK Health Centre and is a researcher at the Canadian Center for Vaccinology. She’s also head of pediatric infectious diseases at the IWK, and serves as co-chairperson of the COVID-19 Vaccine Task Force advising the federal government.

She is also one of the primary investigators behind a new study, ‘Mix and match of the second COVID-19 vaccine dose for Safety and Immunogenicity’ study, referred to as MOSAIC.

“It’s true that we don’t have a huge amount of information on mixing and matching (COVID-19) vaccines, but this is the way that we do that, by citizen participation,” Langley said. “And in Nova Scotia, we’ve been really fortunate that there are people that want to advance science and are willing to help us do that.”

As supplies of Pfizer have decreased in recent weeks and Moderna shipments have increased, the province’s chief medical officer of health, Dr. Robert Strang, has urged Nova Scotians not to wait for a specific vaccine. They are being advised to take whichever they can get first, in line with recent recommendations by the National Advisory Committee on Immunization (NACI).

Despite this, it appears some people are holding out for Pfizer rather than nabbing a Moderna dose they could access much more quickly.

“Whenever there is a big public health program, there are expected interruptions to vaccine supply,” Langley said. “Ways to deal with that are to use the vaccine you have available, so that’s the mixing and the matching. The other is to extend the time between doses.”

‘Immune responses seem to be very good’

The Canadian MOSAIC study is attempting to answer three questions: how safe and effective is a full COVID-19 vaccination if the first and second doses are 28 days apart versus 112 days apart; how safe and effective is a two-dose COVID-19 vaccine schedule when different vaccines are used for the first and second dose; and how long does protection from the virus last in the above conditions.

“There are studies that are being reported every few days on the experience (of vaccine mixing and matching) so far in other countries like Germany, Spain, and the United Kingdom,” Langley said.

“There’s no concerning information arising from those studies and so far, the immune responses seem to be very good.”

Langley said despite this, it’s important to have Canadian-specific data. The MOSAIC study will include about 1,200 participants from across the country, including Halifax. About 100 people have already signed up.

The interchangeability of vaccines isn’t a new concept. The National Advisory Committee on Immunization (NACI) notes that similar vaccines from different manufacturers are routinely used in the face of changing supply or public health programs.

Examples include different vaccine products in a vaccine series for Hepatitis A, monovalent Hepatitis B, Influenza, Measles, Mumps, Rubella (MMR), Meningococcal conjugate vaccines and vaccines used for routine primary immunization series of diphtheria toxoid, tetanus toxoid, pertussis, poliomyelitis and Haemophilus influenzae type b (DTaP-IPV-Hib).

General vaccine principles indicate that to be considered interchangeable, vaccines should be authorized with the same indications and with similar schedules, for the same population, contain or produce comparable type(s) of antigen, and be similar in terms of safety, reactogenicity, immunogenicity and efficacy.

Langley points to influenza as one of many examples where a different vaccine might be used against the same pathogen. Each year, you may receive a vaccine from a different company, but they’re similar products and both protect against influenza.

“Your immune system is very skilled at recognizing the antigen–the part you’re trying to create an immune response against — so that has turned out to be a safe and effective thing to do,” she said.

Early data suggests people do get a good immune response with a different COVID-19 vaccine for their second dose. In an article published by NPR last month, Helen Fletcher, a professor of immunology at the London School of Hygiene & Tropical Medicine in the UK, said there’s a practical and valid scientific reason for mixing two different kinds of vaccine. She noted you can get a better immune response than if given the same vaccine twice.

“Basically, all vaccines work by showing people’s immune systems something that looks like an invading virus but really isn’t. If the real virus ever comes along, their immune systems will recognize it and be prepared to fight it off,” reporter Joe Palca wrote.

“Using two different vaccines is a bit like giving the immune system two pictures of the virus, maybe one face-on and one in profile.”

Langley described this “two pictures” analogy as a great explanation. She added that when you get a second dose of a different vaccine, there does seem to be a bit more “reactogenicity.”

“Normally we would expect a sore injection site, some aches and pains, feeling off for the next two days. But those do resolve quickly and they are indications that your immune system is engaged and responding and will make a vibrant immune response,” Langley said.

“The data so far if you’ve had AstraZeneca and then mRNA (a Pfizer of Moderna vaccine) is that your responses seem to be a little bit higher then. But the efficacy data shows that they’re very close in terms of protection against hospitalization, severe disease, or death.”

Langley described the up-to-date information around reported side effects of COVID-19 vaccination provided regularly to Canadians as “a new age of timely transparency.” She said the federal government’s weekly vaccine safety report allows the public to know what the experts know.

“The public health agency is not saying ‘We’re going to digest this and only tell you what we think you need to know,’” she said.

“You can see the things that occur that may or may not be related, and then you can see whenever they have a concern about something, whether it’s clotting events or excessive fever. Whatever it is, they report that.”

Vaccines ‘one of the chief pillars of good health’

The majority of the MOSAIC study’s participants — about 80% — will have already received their first dose of an approved vaccine. They’ll receive their second randomized dose in the study setting. The remaining 20% of participants will receive their first and second randomized doses during study visits.

While AstraZeneca won’t be offered in the MOSAIC study, those who had AstraZeneca as a first dose can participate and will receive a randomized second dose of either Pfizer or Moderna in a study setting.

Echoing public health officials across the country, Langley said it’s critical that those who can be vaccinated do so to protect themselves and others, adding that the science of vaccines is solid.

“They’re one of the chief interventions that humanity has come up with to save lives, prevent disability, to prevent long term chronic illness. If you think of the newborns that have been harmed by rubella, who were prior to vaccines born with heart disease and blindness, vaccines have been something that’s really altered our life,” Langley said.

“Along with clean water and recognition that cigarettes cause lung cancer, they’re really one of the chief pillars of good health.”

Latest data from the Public Health Agency of Canada shows the number who’ve had two different COVID-19 vaccines is still relatively low. Graphic: Public Health Agency of Canada
Latest data from the Public Health Agency of Canada shows the number who’ve had two different COVID-19 vaccines is still relatively low. Graphic: Public Health Agency of Canada

The MOSAIC study is expected to provide more information about how long immune responses last when people are given a different second dose of COVID-19 vaccine versus two doses of the same vaccine.

Those who participate are also likely to be offered the chance to participate in a year-two vaccine if more doses of COVID-19 vaccine are needed in future.

Funding for the MOSAIC study was announced on May 20, and the Halifax study site gave its first injections earlier this month. People interested in participating can consult MOSAIC’s website for more details.

“Studying vaccine programs as they roll out allows us to improve them, so this is a chance for people in the public to improve vaccine programs and contribute to the betterment of the program overall in the years to come,” Langley said.

Whether or not they participate in the MOSAIC study, she encourages people to get vaccinated against COVID-19 to help Canada return to a more “community-based” life.

“We need each other,” she said. “Isolation is not a good long-term plan.”

Yvette d'Entremont

Yvette d’Entremont is a bilingual (English/French) journalist and editor, covering the COVID-19 pandemic and health issues. Twitter @ydentremont

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