What happens if you mix COVID-19 vaccines, receiving a first dose of one jab and a second dose of a different inoculation? Scientists want to know.

Why? Because mixing doses might give governments more flexibility to stretch available vaccine supplies across populations, ensuring more people get vaccinated and doses won’t go to waste. There’s also speculation that, at least for some of the vaccines, receiving a second dose of a different vaccine might produce a stronger immune response.

Finally, if COVID-19 becomes endemic —as many experts think is likely— and people need frequent booster shots to retain immunity, then it is possible people will need to receive boosters of a different shot than they received initially.

The U.K. government is sponsoring one of the earliest and largest clinical trials of this kind of mixing, which experts call “heterologous dosing.” And the very first preliminary results from that study are in: People who received the Pfizer vaccine followed by the AstraZeneca jab or vice versa, were more likely to experience uncomfortable reactions to the second dose than people who received two doses of the same vaccine, researchers at the University of Oxford reported in a research note published in the peer-reviewed medical journal The Lancet.

These reactions included flu-like symptoms, such as fever, muscle aches, lethargy, and a general feeling of being unwell. The scientists said that none of these side effects were severe, describing them as “mild to moderate.” They also said the symptoms were short-lived, lasting at most a few days and that there had been no other safety concerns so far with mixing the Pfizer and AstraZeneca vaccines.

More frequent and pronounced reactions could lead to higher rates of absenteeism from work on the day after the second inoculation. This could be an important consideration, especially when governments were vaccinating healthcare workers, according to Matthew Snape, a University of Oxford professor of paediatrics and vaccinology who is leading the mixed dose clinical trial. The scientists also cautioned that the study only included people over the age of 50 and that it is possible that younger people might react differently….

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