How effective is a single vaccine dose against Covid-19?

 https://www.bbc.com/future/article/20210114-covid-19-how-effective-is-a-single-vaccine-dose

 According to Pfizer data published in December 2020, the Pfizer-BioNTech vaccine is roughly 52% effective after the first dose. Out of 36,523 participants in the phase three trial – the final stage of testing where people either received two full doses, 21 days apart, or a placebo – who had no evidence of existing infection, 82 people in the placebo group and 39 in the vaccine group developed Covid-19 symptoms.