Vaccination Unavailability or Delayed Vaccination: Two Plausible Pandemic Scenarios Applied to the COVID-19 Outbreak in Puerto Rico

This study examines two plausible pandemic scenarios, vaccine unavailability and delayed vaccination, using deterministic compartmental models calibrated to the COVID-19 pandemic in Puerto Rico. An SEIsIaHRD model for the pre-vaccination phase and an SVEIsIaHRD model (susceptible, vaccinated, exposed, symptomatic and asymptomatic infectious, hospitalized, recovered and cumulative deaths) for the vaccination phase. Each infected compartment has a single exit rate partitioned by outcome fractions, separating mean sojourn times from clinical outcome proportions. Parameters were calibrated against Puerto Rico dashboard data at two checkpoints: the first day of vaccination (15 December 2020) and the official end of the pandemic (11 May 2023). Local stability was established by Jacobian analysis, and an alternative procedure gave the basic (Ro) and vaccinated (Rv) reproduction numbers without the standard next-generation matrix method. Simulations forecast that approximately 96% of the population would be vaccinated, a cumulative 3,146,734 individuals differing from the reported figure by 0.32%, and reproduced the reported daily incidence at the pre-vaccination checkpoint to within 10.3%. A vaccine with 87% effectiveness reduced cumulative deaths from 45,124 to 5608, and long rollout delays nullify most of the benefit. With vaccination, Rv = 0.16, the disease dies out; without it, Ro = 1.22, and it becomes endemic.