Much of the withheld information could help state and local health officials better target their efforts to bring the virus under control. Detailed, timely data on hospitalizations by age and race would help health officials identify and help the populations at highest risk. Information on hospitalizations and death by age and vaccination status would have helped inform whether healthy adults needed booster shots. And wastewater surveillance across the nation would spot outbreaks and emerging variants early.

Without the booster data for 18- to 49-year-olds, the outside experts whom federal health agencies look to for advice had to rely on numbers from Israel to make their recommendations on the shots…

Last year, the agency repeatedly came under fire for not tracking so-called breakthrough infections in vaccinated Americans, and focusing only on individuals who became ill enough to be hospitalized or die. The agency presented that information as risk comparisons with unvaccinated adults, rather than provide timely snapshots of hospitalized patients stratified by age, sex, race and vaccination status.

But the C.D.C. has been routinely collecting information since the Covid vaccines were first rolled out last year, according to a federal official familiar with the effort. The agency has been reluctant to make those figures public, the official said, because they might be misinterpreted as the vaccines being ineffective.