What do we know so far about mixing and matching Covid-19 vaccines?

In an ideal world, it would not 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 studies 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.

The study, published in The Lancet in May, also took into account the different dosing regimens, with intervals ranging from 28 days to 84 days. This is particularly key for countries like the United Kingdom, 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.

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.

In India, too, an accident with administering vaccines has led to the discovery that mixing vaccines may be safe.

Covaxin-Covishield mixing

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 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 WHO says

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 has warned against individuals mixing vaccines on their own. “It is 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 August 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.

Fixing supply chain

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 24 lakh 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.

This article first appeared on Quartz.