Anthony Fauci, President Biden’s chief medical adviser, said on Monday that a COVID-19 vaccine booster specific to the BA.5 omicron subvariant — which is currently dominant in the U.S. — is the “best guess” for dealing with the virus this fall amid the ever-evolving coronavirus pandemic.

Appearing on Hill.TV’s “Rising,” Fauci reiterated that it is difficult to predict how SARS-CoV-2 will mutate and noted that the U.S. is currently in a “BA.5 mode,” with roughly 80 percent of cases caused by the subvariant.

The BA.5 subvariant is able to better evade the immune protections offered by vaccines and prior infections than other strains. Its rise has led health experts to call for the development of next-generation COVID-19 vaccines that would be “variant proof.” Read more here.