While research shows that the Pfizer-BioNTech vaccine is fully effective only after the recommended two doses, a December 2020 study found that “in the interval between the first and second doses, the observed vaccine efficacy was 52%.”
That same month, the U.K.’s official scientific-advisory panel published a study that found that a single dose of the vaccine was 89% effective 15-21 days after vaccination.
In January, a World Health Organization adviser reported a 33% drop in COVID-19 positivity rates among 200,000 Israelis who had been vaccinated 14 days prior, suggesting a lower single-dose efficacy rate. However, the Israeli health ministry said this assessment was “out of context and, therefore, inaccurate.” A London medical-school professor stated that the observation was not “sufficient” to invalidate previous findings.