Hall et al conducted a prospective cohort study to investigate the effectiveness of the BNT162b2 in healthcare workers who resided in the United Kingdom and were over 18 years of age. The participants were divided into two groups, the positive cohort and the negative cohort. The positive cohort consisted of individuals who had a positive antibody test or a history of infection which was indicated by previous positivity of antibody or PCR tests. The negative cohort consisted of participants who had a negative antibody test with no previous positive test. The sample size was of 22,324 individuals with 35% assigned to the positive cohort and 65% assigned to the negative cohort. The research team utilized a piecewise exponential hazard mixed-effects model to conduct their analysis. Upon conducting their analysis, the research team found that a single dose of BNT162b2 vaccine showed an effectiveness of 70% after the first dose and 85% 7 days after the two doses. These results show that the vaccine can prevent both asymptomatic and symptomatic infection in adults, as well as effectiveness against the dominant variant at the time of the study, B1.1.7.