A health-care worker in Nonthaburi, Thailand, examines a vial of the Pfizer–BioNTech COVID-19 vaccine.Credit: Peerapon Boonyakiat/SOPA Images/LightRocket via Getty Study after study has shown that people who receive two different COVID-19 vaccines generate potent immune responses, with side effects no worse than those caused by standard regimens. But now, for the first time, researchers have […]