People in both groups who had a history of one of the 26 health conditions in the previous year were excluded from the study — an attempt by the researchers to consider medical issues that patients developed only after they had Covid.

The study, which involved patients seen at health facilities that use a record system managed by Cerner Corp., a large medical data company, said the Covid patients included people admitted to hospitals, seen in emergency departments or diagnosed in an outpatient setting. The researchers did not indicate how many patients were in each group, one of several limitations of the study’s findings.

Between 30 days and 365 days after their coronavirus diagnosis, 38 percent of the patients experienced one or more new health problems, compared to 16 percent of the non-Covid patients, the study said. The younger age group, 18-to-64, was somewhat less likely to have those problems — 35 percent developed long Covid issues, compared with 15 percent of uninfected people. In the 65-and-older group, 45 percent had new health conditions, compared with 19 percent of uninfected people.

Based on those percentages, the study authors calculated that nearly 21 percent of the younger group and nearly 27 percent of the older group developed health problems that could be attributed to long Covid.

The study did not look at the vaccination status of the patients and did not report characteristics like race, ethnicity, sex or geographic location. It also did not identify which coronavirus variants were linked to each case.

The C.D.C. authors concluded that post-Covid conditions might “affect a patient’s ability to contribute to the work force and might have economic consequences for survivors and their dependents.” They added that “care requirements might place a strain on health services” in “communities that experience heavy Covid-19 case surges.”

Dr. Al-Aly said he agreed that people who had Covid should be medically evaluated for potential new health problems.

“Now that we are in possession of knowledge that Covid-19 can lead to serious long-term consequences,” he added, “we need to develop additional tools to reduce the risk of long Covid.”