The COVID-19 vaccine by Oxford University and AstraZeneca was found to be 76% effective against symptomatic infection for three months following a single dose in a study. The vaccine efficacy increased if the second shot is delayed.

Chief Investigator of the Oxford Vaccine Trial, Andrew Pollard, stated that the 12-week interval between the two doses appeared to be the optimal approach, and that people are protected from 22 days following a single dose. Vaccine efficacy after a single dose from day 22 to day 90 following vaccination was found to be 76%. Modelled analysis suggested that the protection did not decrease during these 3 months. Additionally, the efficacy of the vaccine was 82.4% with a gap of 12 or more weeks to the second dose. On the contrary, the efficacy was 54.9% for those where the booster was administered less than 6 weeks after the first dose…(Reuters, February 2, 2021)