Mixing COVID-19 vaccines appears to boost immune responses

Faced with short supplies of COVID-19 vaccines and unforeseen side effects, some countries have adopted an unproven strategy: switching shots midstream. Most authorized vaccines require two doses administered weeks or months apart, but Canada and several European countries are now recommending a different vaccine for the second dose in some patients. Early data suggest the approach, born of necessity, may actually be beneficial. In three recent studies, researchers have found that following one dose of the vaccine made by AstraZeneca with a dose of the Pfizer-BioNTech vaccine produces strong immune responses, as measured by blood tests. Two of the studies even suggest the mixed vaccine response will be at least as protective as two doses of the Pfizer-BioNTech product, one of the most effective COVID-19 vaccines. Only a few of the potential vaccine combinations have been tested. But if mixing vaccines proves safe and effective, it could speed the effort to protect...