The makers of vaccines for the coronavirus disease 2019 (Covid-19)  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 (WHO) 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.

“When do you pull the trigger?” asked Norman Baylor, a former Food and Drug Administration vaccine chief. “This is a moving target right now.”

FLU OFFERS A MODEL

The WHO and Food and Drug Administration are looking to the global flu vaccine system in deciding how to handle similar decisions about COVID-19 shots.

Influenza mutates much faster than the coronavirus, and flu shots have to be adjusted just about every year. National centers around the globe collect circulating flu viruses and track how they’re evolving. They send samples to WHO-designated labs for more sophisticated “antigenic” testing to determine vaccine strength. The WHO and regulators then agree on the year’s vaccine recipe and manufacturers get to work.

For Covid-19 vaccines, Webby said a critical step is establishing a similar surveillance and testing network to flag the mutations that matter. Today, there’s wide geographic variability in tracking and testing mutated versions. For example, Britain does more testing of the changing viral genome than the US.