ABSTRACT
BackgroundCOVID-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. MethodsWe 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. ResultsWe 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. ConclusionsDesigning 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.
ABSTRACT
BackgroundFour COVID-19 vaccine candidates developed by Pfizer, Moderna, University of Oxford/ Astra Zeneca (also Oxford/ Serum Institute of India) and ICMR/ Bharat Biotech have been granted emergency use authorization in the democratic world following established clinical trial procedures in their respective countries. Vaccination of the general public is expected to begin in several weeks. We consider the question of whether people who have received the vaccine can be selectively and immediately cleared to return to normal activities, including hassle-free travel. MethodsWe use a delay differential equation model developed previously by our group to calculate the effects of vaccinee "immunity passports" on the spreading trajectories of the disease. We consider default virus strains as well as high-transmissibility variants such as B1.1.7 in our analysis. ResultsWe find that with high vaccine efficacy of 80 percent or greater, vaccinees may be immediately cleared for normal life with no significant increase in case counts. Free travel of such vaccinees between two regions should not jeopardize the infection control performance of either. At current vaccine administration rates, it may be eight months or more before COVID-19 transmission is significantly reduced or eliminated. With lower vaccine efficacy of approximately 60 percent however, social as well as travel restrictions for vaccinees may need to remain in place until transmission of the disease is eliminated. ConclusionsDesigning 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 drive with a view to analysing socio-demographic determinants if any of efficacy, and optimizing distribution accordingly. A speedy and efficacious vaccination drive will provide the smoothest path out of the pandemic with the least additional caseloads, death toll and socioeconomic cost.
ABSTRACT
COVID-19 is caused by a hitherto nonexistent pathogen, hence the immune response to the disease is currently unknown. Studies conducted over the past few weeks have found that the antibody titre levels in the blood plasma of infected patients decrease over time, as is common for acute viral infections. Fully documented reinfection cases from Hong Kong, India, Belgium and USA, as well as credible to anecdotal evidence of second-time cases from other countries, bring into sharp focus the question of what profile the epidemic trajectories may take if immunity were really to be temporary in a significant fraction of the population. Here we use mathematical modeling to answer this question, constructing a novel delay differential equation model which is tailored to accommodate different kinds of immune response. We consider two immune responses here : (a) where a recovered case becomes completely susceptible after a given time interval following infection and (b) where a first-time recovered case becomes susceptible to a lower virulence infection after a given time interval following recovery, and becomes permanently immunized by a second infection. We find possible solutions exhibiting large number of waves of disease in the first situation and two to three waves in the second situation. Interestingly however, these multiple wave solutions are manifest only for some intermediate values of the reproduction number R, which is governed by public health intervention measures. For sufficiently low as well as sufficiently high R, we find conventional single-wave solutions despite the short-lived immunity. Our results cast insight into the potential spreading dynamics of the disease and might also be useful for analysing the spread after a vaccine is invented, and mass vaccination programs initiated.
