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Free keywords:
SIR model, vaccination strategies, vaccine escape
Abstract:
When vaccine supply is limited but population immunization urgent, theallocation of the available doses needs to be carefully considered. Oneaspect of dose allocation is the time interval between the first and thesecond injections in two-dose vaccines. By stretching this interval, more indi-viduals can be vaccinated with the first dose more quickly, which can bebeneficial in reducing case numbers, provided a single dose is sufficientlyeffective. On the other hand, there has been concern that intermediatelevels of immunity in partially vaccinated individuals may favour theevolution of vaccine escape mutants. In that case, a large fraction of half-vaccinated individuals would pose a risk—but only if they encounter thevirus. This raises the question whether there is a conflict between reducingthe burden and the risk of vaccine escape evolution or not. We develop anSIR-type model to assess the population-level effects of the timing of thesecond dose. Trade-offs can occur both if vaccine escape evolution is morelikely or if it is less likely in half-vaccinated than in unvaccinated individ-uals. Their presence or absence depends on the efficacies for susceptibilityand transmissibility elicited by a single dose.
