CHALLENGES IN ESTIMATING THE EFFECTIVENESS OF 2 DOSES OF COVID-19 VACCINE BEYOND 6 MONTHS IN ENGLAND.

Understanding how effectiveness of COVID-19 vaccines changes over time and in response to new SARSCoV-2 variants is crucial to scheduling subsequent doses. A previous study quantified vaccine effectiveness (VE) over six consecutive 4-week periods from 2 to 26 weeks after second dose (1). Waning of hazard ratios (HRs) comparing vaccinated with unvaccinated individuals was approximately log-linear over time, and consistent across COVID-19-related outcomes and risk-based subgroups. To investigate waning beyond 26 weeks and in the omicron era, we extended follow-up to the earliest of 50 weeks after second dose or 31 March 2022.