The United States appears to be in the midst of another biological baton pass between Covid-19 variants. The Omicron lineage BA.2 and its spinoff, BA.2.12.1, drove cases this spring, building into waves of infections in places like the Northeast and parts of California. Now, two other forms of Omicron, BA.4 and BA.5, are eating into the BA.2 group’s dominance. BA.4 and BA.5 are picking up speed because they’re able to evade the body’s antibody response even more so than other variants, meaning they’re very good at establishing infections in people who have some level of protection.