Fig. 1: Participant flow diagram^¥.

^¥Classification into the four groups accounts for PCR test information held at UKHSA and also self-report. ^βEnrolled population considers only children and young people who were invited to respond at 24-month, i.e. those who registered at 3-month (n = 7356), 6-month (n = 10,530) or 12-month (n = 13,126).