For more than a year, the Centers for Disease Control and Prevention has collected data on hospitalizations for Covid-19 in the United States and broken it down by age, race and vaccination status. But it has not made most of the information public. When the C.D.C. published the first significant data on the effectiveness of boosters in adults younger than 65 two weeks ago, it left out the numbers for a huge portion of that population: 18- to 49-year-olds, the group least likely to benefit from extra shots, because the first two doses already left them well-protected. The agency recently debuted a dashboard of wastewater data on its website that will be updated daily and might provide early signals of an oncoming surge of Covid cases. Some states and localities had been sharing wastewater information with the agency since the start of the pandemic, but it had never before released those findings. Two full years into the pandemic, the agency leading the country's response to the public health emergency has published only a tiny fraction of the data it has collected, several people familiar with the data said. But the C.D.C. has been routinely collecting information since the Covid vaccines were first rolled out last year. The agency has been reluctant to make those figures public, [an] official said, because they might be misinterpreted as the vaccines being ineffective. Experts dismissed the potential misuse or misinterpretation of data as an acceptable reason for not releasing it.