Outcome in patients undergoing postponed elective surgery during the COVID-19 pandemic (TRACE II): study protocol for a multicentre prospective observational study oleh Markus W Hollmann, Gert Jan Scheffer, Wolfgang Buhre, Sander van Kuijk, Seppe Koopman, Dianne de Korte-de Boer, Peter G Noordzij, Alice C Werger, Jennifer Breel, Carolien S E Bulte, Bas A in ‘t Veld, Carin G C L Wensing