The Food and Drug Administration has authorized emergency use of two booster shots designed to better protect against widely circulating variants of the coronavirus. The vaccines produced by Pfizer and Moderna are nearly identical to mRNA shots that were first authorized for use in the U.S. in December of 2020, but the new formulations target the BA.4 and BA.5 Omicron subvariants that currently account for nearly all U.S. COVID cases. Vaccine experts say the new boosters remain safe, but it’s far less clear how effective they’ll be. Data submitted by Pfizer and Moderna to the FDA relies on studies in mice, and there’s currently no clinical trial data studying the boosters’ efficacy in humans. A CDC panel of vaccine experts meets today to discuss the boosters. If the agency approves them, as expected, shots will be available, free of charge, at pharmacies, doctors’ offices and community health centers just after the Labor Day weekend.