Covid-19 patients that had to be treated in a hospital, that is ("Individuals recovered from COVID-19 at three public hospitals in Madrid"). Or does "patient" already imply that?
And dropping the "after recovery" goes astray from the central point of the article. That we are mistaken when we think that people have recovered.
should work; it's easy to infer that "after a year" refers to their hospitalisation anyway. If that is still too long: "Less than 20% hospitalised COVID patients symptom free after a year"