1 COVID-19 National Epidemiology Cell, UK Health Security Agency, London, NW9 5DF, UK. Electronic address: feedback.c19epi@phe.gov.uk.
2 Immunisation Division, UK Health Security Agency, London, UK.
3 COVID-19 National Epidemiology Cell, UK Health Security Agency, London, NW9 5DF, UK.
4 Immunisation Division, UK Health Security Agency, London, UK; Paediatric Infectious Diseases Research Group, St George's University of London, London, UK.