Comparison between one and two dose SARS-CoV-2 vaccine prioritisation for a fixed number of vaccine doses

Hill, E. M.; Keeling, M. J.

Background: The swift development of vaccines targeting SARS-CoV-2, which have been shown to generate significant immune responses and offer considerable protection against disease, has been met with worldwide commendation. However, in the context of an ongoing pandemic there is an interplay between infection and vaccination. While infection can grow exponentially, potentially overwhelming healthcare resources, vaccination rates are generally limited by both supply and logistics. With the first SARS-CoV-2 vaccines receiving medical approval requiring two doses, there has been scrutiny on the spacing between doses; an elongated period between doses would allow more of the population to receive a first vaccine dose in the short-term generating wide-spread partial immunity. Methods: Focusing on data from England, we investigated prioritisation of a one dose or two dose vaccination schedule given a fixed number of vaccine doses and with respect to a measure of maximising averted deaths. We optimised outcomes for two different estimates of population size and relative risk of mortality for at-risk groups within the Phase 1 vaccine priority order in England, for different amounts of available vaccine and for different vaccine efficacies. Findings: We find that vaccines offering relatively high protection from the first dose (compared to the efficacy derived from two doses) favour strategies that prioritise giving more people one dose rather than a smaller number two. The optimal mix of one and two doses between the defined priority groups of Phase 1 shows a pattern of returning to give second doses to the highest risk groups as the number of available doses increases. Discussion: While this work highlights that an optimal timing of first and second doses between the Phase 1 priority groups can substantially reduce the overall mortality risk to the population, there also needs to be careful consideration of the precise timing between first and second doses as well as the logistics of vaccine delivery.