https://www.nature.com/articles/d41586-020-00502-w
The two-dose vaccine developed by the University of Oxford, UK, and pharmaceutical firm AstraZeneca in Cambridge, UK, could be given to more people by lengthening the interval between jabs. To test the efficacy of this strategy, Andrew Pollard at the University of Oxford and his colleagues examined clinical-trial data from more than 17,000 people, half of whom received the vaccine and the other half a placebo (M. Voysey et al. Lancet https://doi.org/fwk7; 2021).
The team found that, for intervals greater than six weeks, the longer the gap between jabs, the better the vaccine protected against COVID-19. It was 55% effective in those who received their second dose less than 6 weeks after their first, and 81% effective in those whose second dose was more than 12 weeks after their first. The team also found that a single dose of the vaccine had an efficacy of 76% for the first 90 days after vaccination.