A third of people hospitalized with Covid-19 have "abnormalities" in multiple organs months after getting infected, a UK study said on Saturday, potentially shedding light on the elusive condition of long Covid.

Millions worldwide are estimated to suffer from long Covid, in which a range of symptoms such as shortness of breath, fatigue and brain fog last long after patients first contracted the virus.

Yet much about the condition, including exactly how Covid causes such a wide range of symptoms, remains unknown.

The authors of the new study, which was published in The Lancet Respiratory Medicine journal, said it marks a "step forward" in helping long Covid sufferers.

The study is the first to look at magnetic resonance imaging (MRI) scans of multiple organs — the brain, heart, liver, kidneys and lungs — after being hospitalized with Covid.

It compared the organ scans of 259 adults hospitalized with Covid across the UK in 2020-2021 with a control group of 52 people who never contracted the virus.

Nearly a third of the Covid patients had abnormalities in more than one organ an average of five months after leaving hospital, the study found.

Those hospitalized with Covid were 14 times more likely to have lung abnormalities, and were three times more likely to have abnormalities in their brain, it said. However hearts and livers appeared to be more resilient, the researchers added.