WASHINGTON (BLOOMBERG) - Almost one-fifth of US adults who have had Covid-19 are currently experiencing symptoms of long Covid that persist for at least three months, new federal data show.

The results of a June survey by the US Census shed light on how frequently the virus's symptoms linger beyond the initial period when people are acutely sick.

More than one-third of US Covid patients reported ever having long Covid symptoms, and 19 per cent said they currently were experiencing them.

The collection of symptoms known as long Covid can include fatigue, brain fog, breathing problems, pain or other after-effects.

Scientists are still untangling why Covid-19 symptoms sometimes last for months, and gathering data is complicated because the condition can be difficult to define.

How common long Covid-19 is, and how long symptoms last, could have implications for health-care and disability costs and the health of the workforce.

About 40 per cent of Americans reported ever having Covid in the survey, which was conducted by the Census and analysed by the US Centres for Disease Control and Prevention's statistical arm.

About 1 in 13 of all US adults currently have at least one long Covid symptom, either severe or mild, with women more likely to report lingering effects than men.

The survey is designed to collect data quickly and is subject to several potential sources of error, the CDC said.

An analysis published in April in the Journal of Infectious Diseases suggested symptoms persist in about 43 per cent of patients.