Most patients do not know how they got sick.

More than half of people with COVID-19 have no idea how or where they became infected, underscoring the need for social distancing, more widespread use of masks, and better follow-up of contacts, especially at work.

That’s the conclusion of a new survey by the Centers for Disease Control, released Tuesday, of 350 patients in nine states treated at 11 academic medical centers, including Stanford University and the University of California, Los Angeles.

It was found that 54% of patients were unaware of recent close contact with a patient with COVID-19.

Of the patients who knew the source of their illness, 45% said they were likely infected from close contact with a sick family member, 34% by a co-worker, and 10% by a friend. Others said they were exposed in a healthcare setting, assisted living facility, correctional facility, or by a neighbor or client at work.

Until now, reports on sources of exposure to the COVID-19 virus have focused on so-called congregation settings, such as meat and poultry processing plants and long-term care facilities. And these reports focused primarily on patients who were so ill that they required hospitalization.

This new survey, conducted by telephone, is considered a much more representative image of the individual behaviors and demographic characteristics of patients in the general population, both internal and external patients.

Approximately two thirds – 64% – of these patients were employed. Of these, only 17% worked remotely, through “teleworking”.

“The need for improved measures to ensure safety in the workplace is justified, including ensuring social distancing and the more widespread use of cloth face covers,” the CDC report concluded.

The new findings come at a time of increasing infections in many states, including California, as counties begin to reopen and testing expands.

While an increase in cases was expected, “few expected it to be as rapid and widespread,” said Dr. Robert Wachter, chairman of the UC San Francisco Department of Medicine.

The CDC report is based on telephone surveys of a sample of patients with positive COVID-19 results between April 15 and May 24 who were hospitalized or received outpatient care. They were asked about exposure, defined as being within six feet of someone with a diagnosis of COVID-19, during the two weeks preceding the positive test result or illness.

The average age of outpatients was 42, while the average age of hospitalized patients was 54. Hospitalized patients were less likely to be white and more likely to have an annual household income of less than $ 25,000. Hospitalized patients also had more underlying chronic diseases, such as cardiovascular disease, chronic respiratory disease, or diabetes.

About a third of sick people seen as outpatients reported that their health had not yet returned to normal two or three weeks after testing positive.