The researchers of this study employed a model to quantify the impact of COVID-19 vaccine prioritization. The researchers studied three different areas of variation: performance of the vaccine, susceptibility to infection and infection fatality, and population and policy. The study found that direct vaccination of adults over the age of 60 reduced mortality and years of life lost when COVID-19 transmission was high. In evaluating transmission, assuming that the vaccine is 90% effective, the model found that vaccinated adults aged 20-49 years minimized infections and the vaccination of individuals aged 60 or over always minimized years of life lost. The researchers also studied vaccine prioritization, given that the vaccine is less effective in older populations.Their findings supported the vaccination of adults 60 and older, as long as vaccine efficacy only began to decrease in individuals 70 years old and over. Bubar et al. also evaluated serological testing for vaccine prioritization, including two approaches where vaccination occurred regardless of serostatus and where vaccination depended on a negative serological test. The analysis found that prioritization of seronegative individuals for vaccination allowed for large reductions in cumulative incidence and mortality in areas with higher seroprevalence. The findings of the research study illuminate the importance of utilizing age-stratifying models to evaluate vaccine prioritization strategies.