The early findings from an Oxford University study testing the effect of combining two different vaccines found that mixing doses of two leading COVID-19 vaccines increased patients’ side effects such as fatigue and headaches.
People who got a first dose of Oxford University’s shot followed by American drugmaker Pfizer’s vaccine four weeks later reported more short-lived side effects, most of them mild, researchers reported in The Lancet medical journal. That was also true when the order of the shots was reversed.
Blending two vaccines
Researchers and public health officials are examining strategies such as blending two different shots as many low- and middle-income nations try to figure out how to cope with vaccine scarcity. Assurance that mismatched shots are still safe and effective would make it easier for governments to manage their stockpiles and provide more insight into a combination that’s already used in some countries.
In France, for instance, people who got a first dose of the Oxford vaccine before the government restricted it to older patients are being offered the vaccine developed by Pfizer and BioNTech for their second injection.
“It’s a really intriguing finding and not something that we were necessarily expecting. Whether or not this will relate to an improved immune response, we don’t know yet; we’ll be finding out those results in a few weeks’ time,” said Matthew Snape, an Oxford pediatrics and vaccinology professor who’s leading the trial.
The study didn’t point to any safety issues and the stronger side effects vanished after a few days, he said. The results suggest, however, that mixed dose schedules could result in an increase in work absences the day after immunization, according to Mr. Snape. “You wouldn’t want to immunize a ward full of nurses with a mixed schedule on the same day,” he said.
About 10 percent of participants who got mixed doses reported severe fatigue, for instance, compared with about 3 percent for those inoculated with a single type of vaccine, the research shows. All participants in the study are aged 50 and over. It’s possible that the reactions could be even stronger in younger patients, according to Mr. Snape.
The researchers are also testing a wider dosing interval of 12 weeks between shots and plan to expand the research to encompass vaccines from American drugmakers Moderna and Novavax.