1/3 This report from the Joint Committee on Vaccination and Immunisation (JCVI) is useful
( https://www.cas.mhra.gov.uk/ViewandAcknowledgment/ViewAttachment.aspx?Attachment_id=103741). In summary...

2/3 Published efficacy between doses of Pfizer vaccine was 52.4% (95% CI 29.5-68.4%). This means that roughly speaking efficacy is anywhere between 30% and 70% before serving jab. But most the vaccine failures occurred shortly after 1st dose, the period before...

3/3 ... any immune response is expected. But using data for those cases observed between day 15 and 21, efficacy was estimated at 89% (95% CI 52-97%), suggesting that short term protection from dose 1 is very high from day 14 after vaccination.

Conclusion: if you’ve had 1st Pfizer jab then keep shielding for two weeks, after which you should be pretty well protected. But we don’t know if this fades between 3 and 12 weeks (before 2nd jab). Also we don’t know if you can still transmit, so keep your mask on!!