COVID-19 Spreading dynamics in an age-structured population with selective relaxation of restrictions for vaccinated individuals : a mathematical modeling study

Shayak, B.; Sharma, M. M.; Mishra, A. K.

Background. COVID-19 vaccination of healthcare and other essential workers is underway in many countries while immunization of the general public is expected to begin in the next several weeks. We consider the question of whether people who have received the vaccine can be selectively and immediately permitted to return to normal activities. Methods. We use a delay differential equation model to calculate the effects of vaccinee ``immunity passports'' on the epidemic spreading trajectories. The model incorporates age-structuring to account for children who are ineligible for vaccination, and senior citizens who are especially vulnerable to the disease. We consider consensus strains of virus as well as high-transmissibility variants such as B1.1.7 and B1.351 in our analysis. Results. We find that with high vaccine efficacy of 80 percent or greater, unrestricted vaccinee--vaccinee interactions do not derail the epidemic from a path towards elimination. Vaccinee--non-vaccinee interactions should however be treated with far more caution. At current vaccine administration rates, it may be the better part of a year before COVID-19 transmission is significantly reduced or ceased. With lower vaccine efficacy of approximately 60 percent, restrictions for vaccinees may need to remain in place until the elimination of the disease is achieved. In all cases, the death tolls can be reduced by vaccinating the vulnerable population first. Conclusions. Designing high-efficacy vaccines with easily scalable manufacturing and distribution capacity should remain on the priority list in academic as well as industrial circles. Performance of all vaccines should continue to be monitored in real time during vaccination drives with a view to analysing socio-demographic determinants of efficacy, if any, and optimizing distribution accordingly. A speedy and efficacious vaccination drive augmented with selective relaxations for vaccinees will provide the smoothest path out of the pandemic with the least additional caseloads, death tolls and socio-economic cost.