RT Journal Article SR Electronic T1 Puzzling persisting symptoms after COVID-19 JF ERJ Open Research JO erjor FD European Respiratory Society SP 00709-2021 DO 10.1183/23120541.00709-2021 VO 8 IS 2 A1 Hellemons, Merel E. YR 2022 UL http://openres.ersjournals.com/content/8/2/00709-2021.abstract AB