Risk of COVID-19 epidemic resurgence with the introduction of vaccination passes

Many countries hit by the COVID-19 epidemic consider the introduction of vaccination passes. So far, no thorough impact assessment of vaccination passes and of lower restrictions for their holders has been conducted. Here, we propose the VAP-SIRS model that accounts for susceptible, infected, and recovered subpopulations, also within the group of vaccinated pass holders. The model accounts for imperfect vaccination effectiveness, revaccinations and waning immunity. Different restrictions for pass holders and the rest of the population result in different scenarios of the epidemic evolution, some of which yield unfavourable COVID-19 dynamics and new waves. We identify critical variables that should be considered by policymakers and show how unfavourable outcomes can be avoided using adaptive policies. In particular, while pass holders could initially be allowed large freedoms, the gradual loss of immunity will require either increased restrictions for pass holders, or accelerated revaccination. In the long-term, common restrictions for both the pass holders and the rest of the population will have to be kept to avoid epidemic resurgence. Such minimum required restrictions depend on vaccination effectiveness, revaccination rate, waning rate and fraction of never-vaccinated population, and, for realistic combinations of these parameters, range between 29% and 69% reduction of contacts.