“…Online questionnaires were used for collection of demographic and clinical data, and information on SARS-CoV-2 vaccination, as described previously. 4 , 5 Questionnaires to collect data on SARS-CoV-2 breakthrough infections were sent to participants at fixed timepoints: April 26, Aug 24, and Dec 10, 2021, 4 and March 10, 2022. Breakthrough infections were classified as delta or omicron SARS-CoV-2 infections on the basis of publicly available data on the predominant circulating SARS-CoV-2 variant in the Netherlands at the time ( appendix p 4 ).…”