1. The initial doses of the Pfizer and AstraZeneca vaccines were 91% and 88% efficacious in reducing COVID-19 hospitalizations one month after vaccination.
2. Combined vaccine efficacy of the Pfizer and AstraZeneca in preventing hospital admission due to COVID-19 among those aged 80 years and older was approximately 83%.
Evidence Rating Level: 2 (Good)
Study Rundown: At the time of this publication, the COVID-19 pandemic continues to burden healthcare systems worldwide. In many countries, the demand for vaccines surpasses the available supply. To work around the low vaccine availability, some regions have opted for a first-dose mass vaccination program with a delayed second dose rather than the recommended two-dose regimen with vaccines given three weeks apart. However, the long-term efficacy of various COVID-19 vaccines after a single dose is still largely unknown. In this observational study, the effectiveness of a first dose of either the BNT162b2 mRNA (Pfizer-BioNTech) or ChAdOx1 nCoV-19 (Oxford-AstraZeneca) vaccine in preventing hospital admissions was investigated. This study found that the Pfizer and AstraZeneca vaccines were approximately highly effective in reducing COVID-19-related hospitalizations at approximately one month after the first dose. Overall, this study reports the effectiveness of a single dose mass vaccination program in over one million vaccinated individuals. Nevertheless, the observational nature of this study inclines the results to be confounded by ecological and temporal effects.
In-Depth [prospective cohort]: This study utilized a prospective cohort structure to enroll 1,331,993 subjects between Dec 8, 2020, and Feb 22, 2021. Notably, all those having previously tested positive for COVID-19 were excluded. Treatment exposure was defined as receiving a single dose of either the BNT162b2 mRNA (Pfizer-BioNTech) or ChAdOx1 nCoV-19 (Oxford-AstraZeneca) vaccine between Dec 8, 2020, and Feb 22, 2021. The main outcome was hospital admissions due to COVID-19, or any hospital admission within 28 days of a positive COVID PCR test between Dec 8, 2020, to Feb 22, 2021. Cox models and Poisson regressions were utilized with respective covariates (e.g. comorbidity, smoking status, socioeconomic status etc.); further analyses were utilized to assess certain age groups (18-64, 65-79, >80 yo). A downturn in admissions was observed before the vaccination rollout however further modeling attempted to assess the isolated effects. Vaccine effect at 28-34 days after vaccination led to a reduction in COVID-19 hospital admissions (Pfizer 91% [95% CI 85-94], AstraZeneca 88% [95% CI 75-94]). In a pooled analysis of both vaccines, similar findings were observed among patients aged 18-64 years (92%, 95% CI 82-97), 65-79 years (93%, 95% CI 73-98), and those aged 80 years or older (83%, 95% CI 72-89).
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