Super simple version (underestimates efficacy)
- UK vaccinated ~86% adults (1st dose)
- If vaccines did nothing you'd expect split to be 86 / 14 = 6.1 x more vaxxed than unvaxxed.
- Observed 40 / 60 = 0.67
- So vaccines have "squished" 6.1 down to 0.67; 0.67/6.1 = 89% efficacy.

— Paul Mainwood (@PaulMainwood) July 20, 2021