WASHINGTON — 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.

“It’s not really something you can sort of flip a switch, do overnight,” cautioned Richard Webby, who directs a World Health Organization flu center from St. Jude Children’s Research Hospital.

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.

The good news: Many of the new COVID-19 vaccines are made with new, flexible technology that’s easy to upgrade. What’s harder: Deciding if the virus has mutated enough that it’s time to modify vaccines — and what changes to make.