In a meeting Sunday with high-ranking health officials, Walensky said that she and the advisers want to scrutinize the drug companies’ data and might not agree to such a broad recommendation, said an official close to her who spoke on the condition of anonymity to describe a private meeting. It’s extremely rare for the CDC to overrule the recommendations of its advisers.

“She wants to see the data so she can formulate her decision,” the person said, referring to Pfizer’s most recent request to the FDA. “Someone took that as her reservation. That’s not her reservation. She was pointing out that they might not go along. She knows no matter what, the decision is hers.”

Walensky’s skittishness reflects the attitude of some CDC officials and advisers who are not convinced that young, healthy people need additional protection, especially since the Pfizer-BioNTech and Moderna vaccines have been linked to a rare cardiac side effect in male teenagers and young men. They also note that focusing on boosters may distract front-line personnel from the more critical effort to defeat the pandemic, which is to get the first shots to unvaccinated people.