What we know so far about mixing Covid-19 vaccines

Vaccine shopping.
Vaccine shopping.
Image: REUTERS/Dado Ruvic/Illustration
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In an ideal world, it wouldn’t matter which brand or kind of Covid-19 vaccine one took.

Scientists are now fast approaching that ideal situation, studying the effects of taking different doses of the Covid-19 vaccines available at the moment. The promising news is that most combinations have thus far reported no severe side effects and may even be better than taking two doses of the same vaccine.

Several clinical trials are currently underway for mixing Pfizer’s mRNA shot with AstraZeneca’s viral vector vaccine. A UK-based safety study of over 800 participants, for instance, found that those who received the first dose of the AstraZeneca vaccine (known as the “prime” dose) tolerated Pfizer’s shot as their second dose (called the “boost”) quite well. These also found that a mix of AstraZeneca and Pfizer shots elicited a higher antibody response that taking two doses of the same vaccine.

The study, published in The Lancet in May, also took into account the different dosing regimens, with intervals ranging from 28 to 84 days. This is particularly key for countries like the UK, Canada, and India, where the AstraZeneca vaccine is being administered with a gap of up to 16 weeks.

Beyond safety, a Spanish study found that AstraZeneca vaccinees actually benefited from taking Pfizer’s as their second dose, generating higher levels of antibodies than with shots of the same vaccine.

In the Philippines, researchers are studying the combination of CoronaVac, developed by Beijing’s Sinovac, with each of the six vaccines currently approved in the country. Similarly, Moscow-based Gamaleya Research, which developed the Sputnik V vaccine, is studying the effects of mixing it with the Astrazeneca vaccine.

Given that countries like the UK and Canada have introduced mixed-vaccine doses, several countries are now also recognising travelers who got different doses as fully vaccinated, eligible for traveling without quarantine.

This is not entirely surprising given that mixing vaccines—a practice known as heterologous prime-boost—has been tested out with the Diphtheria, Tetanus, and Pertussis vaccine, among several others.

Booster shots in the US

The US has been hesitant to recognise or recommend mixing vaccines. It also does not currently recognise travelers who took mixed doses as fully vaccinated.

The Centers for Disease Control and Prevention recently approved booster shots for people over the age of 65, and for other age groups with underlying medical conditions or sustained risk of exposure to Covid-19. But this booster shot is currently restricted to those who took the Pfizer shot.

But its Food & Drug Administration is now considering booster shots for the Johnson & Johnson and Moderna vaccines, and the potential benefits of mixing different brands—or even types—of vaccines for the booster dose.

India, too, does not have a programme for mixing doses, but an accident with administering vaccines has led to the discovery that it may be safe.

Mixing Covaxin and Covishield vaccines

In May, 20 people in the state of Uttar Pradesh were accidentally administered doses of India’s homegrown Covaxin six weeks after they took Covishield, the local make of the AstraZeneca vaccine, as their first dose.

This alarming accident led the Indian Council of Medical Research (ICMR) to study the immune response and side effects in 18 out of the 20 people. It then compared these to a group of people who received two doses of either Covaxin or Covishield.

Compared to the group that received doses of the same vaccines, the Covishield-Covaxin mix-and-match group had higher neutralising antibodies, according to a paper by the ICMR that is yet to be peer-reviewed. India’s drugs authority has now approved a full-length study into a heterologous prime-boost of the two vaccines that make up most of India’s Covid-19 immunisation programme.

What the WHO says about mixing vaccines

As international borders open up, and once drug companies begin applying for full licences, vaccines will be available in private clinics and centres outside of the national immunisation programmes. This could potentially spur a dangerous trend of vaccine shopping.

For this reason, the World Health Organization (WHO) has warned against individuals mixing vaccines on their own. “It’s a little bit of a dangerous trend here,” WHO’s Soumya Swaminathan told an online briefing on July 12. “It will be a chaotic situation in countries if citizens start deciding when and who will be taking a second, a third and a fourth dose.”

She later clarified in a tweet that public health agencies could take a call based on available data whether to allow mixing two particular vaccines or not.

On Aug. 10, the WHO also cautioned against the quick adoption of mixing vaccines in an interim statement. “While these studies are encouraging, they require cautious interpretation given the limited sample sizes and lack of follow up, especially related to safety data, and the uncertain relevance of immunological readouts in relation to clinical impact,” it said.

But if proven to be safe and beneficial, the ability to mix and match Covid-19 vaccines could have several other benefits.

Mixing the Moderna and Pfizer vaccines

Canada is among the first countries actively trying to manage its vaccine supply issues by allowing the mixing of vaccines. For starters, those in the country who had taken the Pfizer shot were asked to take Moderna’s mRNA vaccine for their second dose. This has helped Canada overcome a supply bottleneck that delayed 2.4 million doses of Pfizer in June.

Taking it even further, on June 17, the Canadian National Advisory Committee on Immunization recommended those who received the AstraZeneca first dose should preferably take either the Pfizer or Moderna mRNA vaccine for their second dose. This decision was taken in light of the rare clotting events reported with the AstraZeneca vaccine. Canadian prime minister Justin Trudeau, too, took Moderna as his second dose on July 2 after the AstraZeneca first dose.

For countries in Asia and Africa, especially those heavily dependant on Covax and international vaccine donations, mixing vaccines could potentially ease the delay in supplies. Countries have currently been hit because of a low supply of the AstraZeneca vaccine, with many manufacturers including the Serum Institute of India running behind on their delivery timelines.