What you need to know about mixing COVID vaccines

As the highly transmissible Delta variant of the coronavirus spreads in dozens of countries around the world, several governments are mixing vaccines in an attempt to boost their inoculation drives.

Mixing vaccines means administering one brand of vaccine for a patient’s first shot, followed by a vaccine made by a different manufacturer for the second dose. Proponents of the policy believe it can increase the speed and effectiveness of vaccination campaigns.

Several ongoing studies are investigating the effects of mixing coronavirus shots. Data has been released from mixed trials in Spain and the United Kingdom, which suggest that mixing vaccines leads to a strong immune response and sometimes outperforms two doses of the same vaccine.

In Germany, a third study also revealed that the immune response of mixing coronavirus doses was better than two AstraZeneca shots and as good as or better than receiving two doses of the Pfizer vaccine.

Which countries are mixing vaccines?

Several countries including Bahrain, Bhutan, Canada, Italy, South Korea, Thailand, and the United Arab Emirates have begun mixing vaccines as policy.

The practice was quietly authorised in January by the Public Health of England when vaccine supplies were limited.

In the same month, US media reported that the Centers for Disease Control and Prevention in the US relaxed its recommendations authorising mixing “in exceptional circumstances”.

In March, several countries paused their vaccine drives amid concerns of extremely rare blood clots associated with the Oxford-AstraZeneca vaccine.

As a response, in some countries, healthcare workers were authorised to administer a different vaccine for some patients’ second shot who received the AstraZeneca jab for their first shot.

Dr Gloria Taliani, professor of infectious disease at Sapienza University of Rome, told Al Jazeera that mixing has been commonplace when treating other diseases in the past.

“We’ve used different vaccines [when we treated] other diseases and we don’t care if the second dose is a different vaccine compared to the first one, or if the boosting dose is a different one.”

Dr Taliani noted that there might be some questions since this is the first time mRNA vaccines have been used to protect against infectious diseases but said there were no biological reasons that suggest mixing could be dangerous.

“There is no biological reason why vaccines that use a different stimulus to the immune system could be harmful to any person,” she explained.

Several world leaders have mixed vaccines in recent months. German Chancellor Angela Merkel, 66, received a second dose of the Moderna vaccine after receiving a first dose of AstraZeneca.

In Italy, Prime Minister Mario Draghi, 73, switched to Pfizer for his second dose after having received an AstraZeneca shot. Canadian Prime Minister Justin Trudeau also switched vaccines, receiving a Moderna shot after an AstraZeneca one.

Is mixing COVID vaccines effective?

The University of Oxford’s Com-COV trial, involving more than 800 volunteers, investigated the efficacy of either two doses of AstraZeneca, Pfizer, or one of them followed by the other.

According to the results, mixed schedules involving the Pfizer vaccine and AstraZeneca shot generated a strong immune response against the virus.

The results of the study suggest that the order of the vaccines made a difference, with AstraZeneca followed by Pfizer “inducing higher antibodies and T-cell responses than Pfizer followed by AstraZeneca”.

The T-cells stimulate antibody production and help combat the virus-infected cells. The research also showed that two doses of Pfizer produced the highest level of antibodies.

Both of the mixes generated better results than the still very effective two-dose AstraZeneca vaccines.

Separately, in May, a Spanish study involving more than 600 volunteers found that an AstraZeneca followed by Pfizer was more effective than two AstraZeneca doses.

However, experts say there is a lack of sufficient clinical data to fully determine whether mixing is effective.

Dr Anna Blakney, an assistant professor in the Michael Smith Laboratories and School of Biomedical Engineering at the University of British Columbia, was part of a team that conducted a trial that looked at mixing an mRNA vaccine and the AstraZeneca viral vector vaccine.

“What we saw in mice is that combining these two was more effective than either one of them alone,” she said.

“So I think it can work but we don’t have the data yet to say whether this is a really effective regime.”

Is it safe to mix COVID vaccines?

No results from the studies have suggested that mixing leads to severe side effects, but results from the British study suggest that mixing vaccines can lead to an increase in mild or moderate side effects.

The data from the Com-COV study showed that 30 to 40 percent of those who received mixed doses reported fevers after a second dose, compared with 10 to 20 percent of those who did not mix the vaccines.

“The results from this study suggest that mixed dose schedules could result in an increase in work absences the day after immunisation, and this is important to consider when planning immunisation of healthcare workers,” Dr Matthew Snape, associate professor in paediatrics and vaccinology at the University of Oxford, and chief investigator on the trial said at the time.

The ongoing Com-COV trial widened to include the Moderna and Novavax vaccines in April.

Results from the Spanish study found that mild side effects were common and similar to those reported from two doses of the same vaccine.

Meanwhile, the World Health Organization chief scientist Soumya Swaminathan has warned individuals against opting to mix vaccines and said decisions should be left to health agencies.

“Individuals should not decide for themselves, public health agencies can, based on available data,” Swaminathan said in a tweet. “Data from mix and match studies of different vaccines are awaited – immunogenicity and safety both need to be evaluated.”