WHO SAGE roadmap on uses of COVID-19 vaccines in the context of OMICRON and substantial population immunity: an approach to optimize the global impact of COVID-19 vaccines at a time when Omicron and its sub-lineages are the dominant circulating variants of concern, based on public health goals, evolving epidemiology, and increasing population-level immunity, first issued 20 October 2020, updated: 13 November 2020, updated: 16 July 2021, update: 21 January 2022, latest update: 30 March 2023