(Reuters) – Young, previously healthy adults can take weeks to fully recover from even a mild COVID-19 infection, with about a fifth of patients under 35 years reporting not returning to their usual state of health up to 21 days after testing positive, according to the Centers for Disease Control and Prevention (CDC).

A telephone survey across 13 states of symptomatic adults with mild COVID-19 found 35% had not returned to their usual state of health when interviewed two to three weeks after testing, the CDC reported in the Morbidity and Mortality Weekly Report on Friday.

Cough, fatigue and shortness of breath were among the symptoms reported while testing that persisted even weeks later, according to the report.

The findings indicate recovery can be prolonged even in young adults without chronic medical conditions, making a case for public health messaging to target populations that might not perceive COVID-19 as being a severe illness.

Between April 15 and June 25, telephone interviews were done with a random sample of people over 18 years of age who got themselves tested for COVID-19 at an outpatient visit, CDC said.

The interviews were done 14 to 21 days after the test date, and patients were asked about symptoms during testing, whether they had returned to their usual state of health, and if they suffer from a chronic medical condition.

Among 292 people interviewed, 274 reported experiencing one or more symptoms at the time of testing. Among symptomatic respondents who reported not having returned to their usual state of health, 26% were between 18 and 34 years of age, 32% were between 35 and 49 years, and 47% were over 50.

Researchers said social distancing, frequent handwashing, and the consistent and correct use of face coverings in public should be encouraged to slow the spread of COVID-19.