What happens when you mix two vaccines?

A study revealed that mixing doses of two leading Covid-19 vaccines increased patients’ side effects such as fatigue and headaches.

Researchers from the University of Oxford reported in The Lancet medical journal that people who got the first dose of AstraZeneca Plc’s shot followed by Pfizer Inc.’s vaccine four weeks later reported more short-lived side effects, most of them mild.

As many poor and middle-income nations try to figure out how to cope with vaccine scarcity, researchers and public health officials are examining strategies such as blending two different shots.

Not every vaccine can work when mixed, but researchers believe it can be done with those that share the same target — in this case the virus’s spike protein. The mixed regimen is known as a heterologous boost.

In France, people who got a first dose of the Astra vaccine before the government restricted it to older patients are being offered the vaccine developed by Pfizer and BioNTech SE for their second injection.

According to Matthew Snape, an Oxford pediatrics and vaccinology professor who’s leading the trial, mixed dose schedules could result in an increase in work absences the day after immunization.

The research shows that, about 10% of participants who got mixed doses reported severe fatigue, for instance, compared with about 3% for those inoculated with a single type of vaccine.

According to Snape, all participants in the study are aged 50 and over. It’s possible that the reactions could be even stronger in younger patients.