The makers of COVID-19 vaccines are figuring out how to tweak their recipes against worrisome virus mutations — and regulators are looking to flu as a blueprint if and when the shots need an update. … Viruses mutate constantly and it takes just the right combination of particular mutations to escape vaccination. But studies are raising concern that first-generation COVID-19 vaccines don’t work as well against a mutant that first emerged in South Africa as they do against other versions circulating around the world.