Efficacy

This morning I was unable to work out for myself what the efficacy of immunisation might be, but a few minutes with Bing, Wikipedia (reference 1) and the New York Times (reference 2) put that right.

A straightforward formula which has been around for a long time and is snapped above. A straightforward formula which masks what might be large 95% confidence intervals (from trials) and which does not tell one all that much about the effectiveness of a vaccination programme. Getting a good programme can be as hard as getting a good vaccine.

Note that this efficacy is not the same as your percentage chance of not getting the disease if you are vaccinated.

Nor does this formula help with the large number of people who are infected but who are asymptomatic spreaders and whom we don't get to know about.

This is not to say that more efficacy in the sense of the formula above is not, generally speaking, a good thing. But there may be trade-offs with the effectiveness of the programme.

Reference 1: Vaccine efficacy - Wikipedia.

Reference 2: 2 Companies Say Their Vaccines Are 95% Effective. What Does That Mean? You might assume that 95 out of every 100 people vaccinated will be protected from Covid-19. But that’s not how the math works - Carl Zimmer/NYT - 2020. Published Nov. 20, 2020, Updated Dec. 4, 2020.

Reference 3: psmv4: Ackroyd. Containing some thoughts from yesterday about getting the foot soldiers to carry through the programme.