The Role of Population Heterogeneity and Immune Response Strength in Informing Optimal COVID-19 Immunization Strategies

Abstract

The COVID-19 pandemic has resulted in unprecedented disruptions to daily life and global economies. Social-distancing measures have been effective in reducing the incidence of infection, but public health authorities and politicians alike are now pinning their hopes of controlling the virus on newly developed vaccines. Since the supply of vaccines is low, decisions must be made about the optimal strategy of vaccine distribution to mitigate severe cases and transmission of COVID-19. To examine the tradeoffs involved in this decision-making process, I employ a three-pronged SIR(S) model to examine the impact of population-level heterogeneity and immune response strength in determining optimal vaccination prioritization strategies. I will consider a population which has three groups: the first with low risk of severe disease but high intragroup transmission rates; the second with low intergroup transmission rates but high risk of severe disease; and a third with average transmission rates and average risk of severe disease. By varying the relative transmission rates and incidence of severe disease in the vulnerable group, I find that population-level heterogeneity plays a far less significant role in determining the optimal vaccination strategy than does immune response strength. In cases where the immune response evoked by natural infection and vaccination is weak, vaccinating the high-transmission group is most beneficial to reduce the incidence of severe cases; however, where the immune response is strong, vaccinating the most vulnerable group is the optimal strategy. Insights about vaccinal and natural immunity will be important in shaping vaccine distribution policy. Additionally, public health policy must consider equity in vaccine access, as well as social determinants of vulnerability, to best serve vulnerable populations.