COVID-19 COVID-19 pandemic Post-COVID-19 syndrome SARS-CoV-2 SARS-CoV-2 virus age asymptomatic central sensitization clinical values co-morbidity comorbidities cross-sectional study dysnatraemia electrolyte imbalance energy fatigue follow-up general perception of health health-related quality of life heart failure hyperbaric oxygenation improvement inspiratory muscle training long COVID long COVID-19 long COVID-19 symptoms long follow-up long haulers long-term effect lung clearance index