It found they had a number of ongoing health conditions.
The study, published in Nature magazine, examined why some patients suffered long-term ill health after admission to hospital with Covid-19.
Previously it has been speculated that long-term ill health was linked to pre-existing medical conditions, but the Glasgow University research suggested it was instead the severity of the coronavirus infection.
The “Cisco” (Cardiac Imaging in SARS Coronavirus disease) study followed the group of discharged patients for a year, comparing their health to a control group of people with similar characteristics.
Further assessments will be made at the 18-month and five-year marks for the patients.
Researchers found one in eight patients admitted to hospital with Covid-19 have heart inflammation, while inflammation across the body and damage to other organs such as the kidneys was also common.
Principle investigator Professor Colin Berry said: “Covid-19 is a multi-system disease, and our study shows that injury on the heart, lungs and kidneys can be seen after initial hospitalisation in scans and blood tests.
“These results bridge a vital knowledge gap between our current understanding of post-Covid-19 syndromes, such as long Covid, and objective evidence of ongoing disease.
“One of the most important findings of the Cisco study is that it is the severity of a patient’s Covid-19 infection – not their underlying health conditions – that is most closely correlated with the severity of any ongoing health outcomes post-discharge.
“We found that previously healthy patients, without any underlying health conditions, were suffering with severe health outcomes, including myocarditis, post-hospitalisation.
“The reasons for this are unclear, but it may be that a healthy person who is hospitalised with Covid-19 is likely to have a worse Covid infection than someone with underlying health conditions who is hospitalised.”
The research paper said women were more likely to be affected by myocarditis post-Covid, which in turn was linked with lower mental and physical wellbeing.