This is a study done in Italy to determine the prevalence of long covid at one year. The subjects had mild-to-moderate covid and were not hospitalized. They were initially enrolled in the study within three weeks of their positive PCR test. One year later, 86% of them completed a follow-up survey about symptoms. I wouldn't be surprised if those with lingering symptoms were over-represented in those that followed through with the study. But given how high their response rate was, it shouldn't affect their results too much. Any time you are trying to calculate how common something is, it's important to try to avoid selection bias. You don't want to just go online and ask for volunteers for a study of how frequent long covid is, because those with long covid will be more interested in joining your study. So the strategy of signing people up right after they get sick and following up later is a good one.
Their results were that 53% had at least one lingering symptom at one year. The most common long covid symptom was fatigue (27%) and the second most common was loss of smell (22%). 13% were still experiencing shortness of breath, and 9% had muscle or joint pain. 7% had insomnia.
Another study found a similar number for loss of smell at one year. The other numbers are higher than I would have predicted, but their methods for getting them are sound.
They found a slight correlation between being female or having a BMI over 25 and having lingering symptoms. They found no correlation with smoking, alcohol consumption, or comorbidities. There was a strong correlation between number of symptoms during their original illness and persistence of symptoms at one year. This last makes me hopeful that the vaccines will dramatically reduce rates of long covid, since they reduce not only the rates of infection, but the number of symptoms for those who get covid in spite of being vaccinated.
People with long covid rated their quality of life as slightly lower than those without. They also rated their level of depression as slightly higher.