When vaccine supply is limited but population immunisation urgent, the allocation of the available doses needs to be carefully considered. One aspect of dose allocation is the time interval between the primer and the booster injections in two-dose vaccines. By stretching this interval, more individuals can be vaccinated with the first dose more quickly. Even if the level of immunity of these 'half-vaccinated' individuals is lower than that of those who have received both shots, delaying the second injection can be beneficial in reducing case numbers, provided a single dose is sufficiently effective. On the other hand, there has been concern that intermediate levels of immunity in partially vaccinated individuals may favour the evolution of vaccine escape mutants. In that case, a large fraction of half-vaccinated individuals would pose a risk - but only if they encounter the virus. This raises the question whether there is a conflict between reducing the burden and the risk of vaccine escape evolution or not. We develop a minimal model to assess the population-level effects of the timing of the booster dose. We set up an SIR-type model, in which more and more individuals become vaccinated with a two-dose vaccine over the course of a pandemic. As expected, there is no trade-off when vaccine escape evolves at equal probabilities in unvaccinated and half-vaccinated patients. If vaccine escape evolves more easily in half-vaccinated patients, the presence or absence of a trade-off depends on the reductions in susceptibility and transmissibility elicited by the primer dose.
Vaccination strategies when vaccines are scarce: On conflicts between reducing the burden and avoiding the evolution of escape mutants