However, experts say more data and evidence required to establish anything for now 

Younten Tshedup

Owing to the vaccine shortage globally and issues with the supply chain, scientists are exploring if different types of Covid-19 vaccine could be safely mixed for inoculation.

Some of the researchers have found that the ‘mix-and-match’ regimens were not only safe but also effective. Research conducted in Spain involving more than 650 people for the trials found that vaccinating people with both the Oxford-AstraZeneca and Pfizer-BioNTech Covid-19 vaccines produces a ‘potent immune response’ against the SARS-CoV-2 virus.

However, further clinical trials and research are ongoing to collect more data.

A national immunization technical advisory group (NI-TAG) member, Dr Sonam Wangchuk said that biologically, vaccines built on the same platform (type of vaccines) although of different brand names, should work in triggering an immune response in the recipients.

He said that if a person who had received AstraZeneca vaccine, which is a viral vector type as their first priming dose, could get another viral vector vaccine such as Russia’s Sputnik V or the Johnson & Johnson’s Janssen vaccine for their second or booster dose.

“This is a possible option especially when there is a shortage and difficulty in getting the same vaccine for the second dose,” he said. “The only concern would be of major side effects or adverse events following immunization (AEFI) which we hope the ongoing studies would soon answer.”


The research 

During the Spanish research, over 650 people who had already received the first dose of the AstraZeneca vaccine (viral vector) were identified for the trial.

Two-thirds of participants were randomly picked to receive Pfizer-BioNTech’s mRNA-based vaccine some eight weeks after the first dose. A control group of 200 plus people were not given any booster dose.

The research found that the Pfizer–BioNTech booster dose jolt the immune systems of the AstraZeneca-dosed participants. It was learnt that after the second dose, participants began to produce much higher levels of antibodies than they did before, and these antibodies were able to recognise and inactivate SARS-CoV-2 virus in laboratory tests.

Control participants who did not receive a booster vaccination experienced no change in antibody levels.

Known as a heterologous prime and boost immunization , the mixing of two different vaccines for different doses is not being used for the first time. Vaccines against other diseases such as Ebola also use the same technique.


Do vaccines mix?

Except for Johnson & Johnson’s Janssen vaccine, all available vaccines for Covid-19 currently require two doses. The first dose primes the immune system and the second dose, usually administered a few weeks after, boosts it.

Dr Sonam Wangchuk said that the prospect of mixing vaccine doses gave a chance to support vaccine rollouts and potentially boost the immunity provided. However, he said that for Covid-19 the evidence to achieve similar results, for now, was limited.

All the approved vaccines against the SARS-CoV-2 virus use different mechanisms to boost immunity. According to experts, the reason why AstraZeneca and Pfizer vaccines could work together was because of the same mechanism involved to target the coronavirus spike protein, although targeting different parts of the spike.

AstraZeneca’s adenovirus vaccine uses a weakened version of a common cold virus found in chimpanzees to present the spike protein to the immune system, while Pfizer’s mRNA-based vaccine delivers genetic instructions for making the spike protein and encourages human cells to produce it.

Vaccines are described as the vehicles delivering cargo — the vehicles may be different, and they may drop off their payloads by different means, but the cargo remains the same.

Scientists argue that because the cargo is identical, the vaccines should, in theory, work well together.

Scientists at Oxford University in the United Kingdom are testing combinations of the two-dose Covid-19 vaccines made by AstraZeneca, Moderna, Novavax and Pfizer-BioNTech. Smaller trials are also ongoing in Spain and Germany.


How safe is it?

A study conducted in the United Kingdom found that those who were given the mix-and-match regimens experienced higher rates of common vaccine-related side effects, such as fever than those who received two doses of the same vaccine.

Even during the Spanish research, mild side effects were common, which were similar to those seen in standard Covid-19 vaccine regimens and none was deemed severe.

Dr Sonam Wangchuk said that minor side effects were common with any vaccine. “Our only concern for now is if this mixing would trigger major AEFI including disabilities. For us to reach any conclusion, we need more evidence and data.”

Sowai Lyonpo (health minister) Dechen Wangmo said that for now, Bhutan needed to be flexible and not limit itself to only one vaccine.

Edited by Tshering Palden