Vaccination strategies when vaccines are scarce: on conflicts between reducing the burden and avoiding the evolution of escape mutants

Abstract

When vaccine supply is limited but population immunization urgent, the allocation of the available doses needs to be carefully considered. One aspect of dose allocation is the time interval between the first and the second injections in two-dose vaccines. By stretching this interval, more individuals can be vaccinated with the first dose more quickly, which 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 an SIR-type model to assess the population-level effects of the timing of the second dose. Trade-offs can occur both if vaccine escape evolution is more likely or if it is less likely in half-vaccinated than in unvaccinated individuals. Their presence or absence depends on the efficacies for susceptibility and transmissibility elicited by a single dose.

Individual costs and societal benefits of interventions during the COVID-19 pandemic

“…The benefits of vaccination can come in different forms: They can reduce transmissibility to others and they can reduce susceptibility for the vaccinated individual [31]. They can also just 5…”

“…The benefits of vaccination can come in different forms: They can reduce transmissibility to others and they can reduce susceptibility for the vaccinated individual [31]. They can also just reduce the probability of severe disease and not have a direct impact on the course of the pandemic.…”

Individual costs and societal benefits of interventions during the COVID-19 pandemic

“…The benefits of vaccination can come in different forms: They can reduce transmissibility to others, and they can reduce susceptibility for the vaccinated individual ( 34 ). They can also just reduce the probability of severe disease and not have a direct impact on the course of the pandemic.…”

Assessing the impacts of vaccination and viral evolution in contact networks

“…Some of these variants had multiple genetic changes which led to phenotypic changes increasing transmissibility and mortality and had the potential to reduce the effectiveness of vaccines 26 . The mass deployment of highly effective vaccines, while the infection is widespread, may rapidly exert selection pressure on the SARS-CoV-2 virus directed towards mutations that escape the vaccine-induced immune response 24 , 27 . However, the strength of this selection and the likelihood of vaccine escape is unknown at this time.…”