Vaccinating people with both the Oxford–AstraZeneca and Pfizer–BioNTech COVID-19 vaccines produces a potent immune response against the virus SARS-CoV-2, researchers conducting a study in Spain have found. A UK trial of a similar strategy reported safety data last week, and is expected to deliver further findings on immune responses soon. Researchers hope that such mix-and-match COVID-19 vaccination regimens will trigger stronger, more robust immune responses than will two doses of a single vaccine, while simplifying immunization efforts for countries facing fluctuating supplies of the various vaccines. A strong immune response to the mix-and-match strategy is “entirely predictable from the basic immunology”, he adds. Repeated doses of virus-based vaccines such as the Oxford–AstraZeneca one tend to be increasingly less effective, because the immune system mounts a response against the adenovirus.