How effective is just one dose?
A paper published in the New England Journal of Medicine stated that the efficacy of the Pfizer-BioNTech vaccine was 52.4% between the first and second dose (spaced 21 days apart).5 However, in its “green book” Public Health England said that during the phase III trial most of the vaccine failures were in the days immediately after the first dose, indicating that the short term protection starts around day 10.6 Looking at the data from day 15 to 21, it calculated that the efficacy against symptomatic covid-19 was around 89% (95% confidence interval 52% to 97%). Meanwhile, Pfizer has said that it has no evidence that the protection lasts beyond the 21 days.
https://www.bmj.com/content/372/bmj.n18
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