Even as tens of millions of inoculated Americans breathe a collective sigh of relief after receiving either the one or two-dose COVID-19 vaccine, some wonder whether one round of shots is enough, or if they’ll need another—and another.

Scientists don’t yet know how long protection from the current cohort of coronavirus vaccines will last. Since the discovery of the original strain in late 2019, the virus has continued to mutate, yielding variants—similar-but-distinctive versions of the virus with the potential to be more infectious, deadly, and escape the antibody safeguards provided by the existing COVID-19 vaccines. To stay ahead of virus evolution, some vaccine creators are racing to design new shots to beat back variants while working to determine how