FILE - In this Jan. 22, 2021, file photo, empty vials of the Pfizer-BioNTech COVID-19 vaccine are seen at a vaccination center at the University of Nevada, Las Vegas, in Las Vegas. The makers of COVID-19 vaccines are figuring out how to tweak their recipes against worrisome virus mutations — if and when the shots need an update. (AP Photo/John Locher, File)
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