The Joint Committee on Vaccination and Immunisation (JCVI) state that individuals receive high levels of short-term protection from a single dose of vaccine.
For the AstraZeneca vaccine there is data in Information for Healthcare Professionals on COVID-19 Vaccine AstraZeneca on p7 under table 2 showing vaccine efficacy (VE) of 73% from 22 days post dose 1, see Regulatory approval of COVID-19 Vaccine AstraZeneca - GOV.UK (www.gov.uk). The most recent published analysis of the AstraZeneca vaccine trial data shows that vaccine efficacy after a single dose of vaccine from day 22 to day 90 after vaccination was 76%, and modelling indicated that protection did not wane during this 3 month period.
A VE for one dose of the Pfizer BioNTech vaccine has been published as 52%, but included cases in the first 14 day period before most vaccines are effective, and higher levels of protection of 89% from 15-21 days after the first dose can be inferred from data provided in the United States authorisation data published by the Food and Drug Administration (FDA) – see p29 Fig 2 at Emergency Use Authorization for Pfizer-BioNTech COVID-19 Vaccine Review Memo (fda.gov)
The experts on the JCVI believe that these levels will offer high levels of short-term protection from a single dose of vaccine during a 12 week interval between the first and second dose.
Increasing the number of individuals receiving the first dose by extending the dose interval up to 12 weeks means many more will have good protection, without reducing the long term effectiveness of the two dose course, and will prevent many more hospitalisations and deaths than using a 3 or 4 week dose interval. (31/12/20)
The JCVI have published a statement explaining the reason for their advice that can be found here.