The decision comes as the Delta variant of COVID-19 continues to spread throughout the country. New data suggests that fully vaccinated people could carry higher levels of the virus than initially believed, and could infect others.

The CDC also recommended that teachers, staff, students, and visitors in schools be required to wear masks, even if they’d been vaccinated and regardless of the level of the virus in that community.

“The Delta variant is showing every day its willingness to outsmart us,” Dr. Rochelle Walensky, director of the CDC, said at a news briefing. “This is not a decision we at CDC have made lightly.”

It’s a reversal that will likely cause further confusion for people. The CDC maintains that vaccinated individuals transmit the virus in only a small percentage of cases—and that unvaccinated people make up the majority of patients who get the Delta variant. However, mounting evidence that even vaccinated people could transmit the disease is driving the decision.

The Biden administration had previously been concerned about issuing a new mask guidance, as the freedom to stop wearing one has been one of the biggest incentives to get jabbed. But the evidence was too overwhelming. White House press secretary Jen Psaki said the administration supported the CDC’s decision.

Just last week, the CDC said there were no plans to change its guidance, unless there were a significant change in the science. Officials reportedly met Sunday to discuss the new evidence, which led to today’s expected recommendation.

The news follows a recommendation by the American Academy of Pediatrics that all children, including those who already have been vaccinated, wear masks when they return to school this fall. Several cities, including Los Angeles, have also reinstituted indoor mask mandates.