The C.D.C. uses information on hospitalizations to help public health officials better understand the evolving epidemiology of a disease, and to help in the planning and prioritization of health care resources.

On Wednesday, the C.D.C. also announced new guidelines outlining how employees who are considered essential by their employers but who have been exposed to people infected by the virus can go back to work. The guidelines apply to workers who do not feel sick and are able to follow certain precautions.

Such workers can return to work if they take their temperature before heading to their workplace, wear a face mask at all times and practice social distancing while on the job, Dr. Robert Redfield, the C.D.C. director, said at the White House coronavirus briefing. While at work, they should not share headsets or other objects that touch their face, and they should not congregate in break rooms or crowded places, he added.

Dr. Redfield said that employers should send workers home immediately if they developed any symptoms. He also said they should increase air exchange in their buildings, and should clean common surfaces more often. The goal, he said, is to “get these workers back into the critical work force so that we don’t have worker shortages.” In its report on hospitalization data, the agency cautioned that the findings about Covid-19 were preliminary, with the number of cases expected to increase considerably as the virus continues to spread and as testing becomes more available. Also, the data does not reflect the surge of cases during the first week of April, straining hospitals in places like New York, New Jersey, Louisiana and Michigan.

But the numbers reflected trends that were reported from other countries at earlier stages of the outbreak. The C.D.C. received information about possible underlying medical conditions in a subset of 180 patients. Approximately 90 percent of those patients had one or more underlying conditions. The most common of those was hypertension, in 49.7 percent of patients, followed by obesity, chronic metabolic disease (like diabetes), chronic lung disease (like asthma) and cardiovascular disease.