Thread by @PJACaytonPhD

Note: I am a stats guy. The learning that I will share is based on interactions with experts far more knowledgeable than me (like @darwinbandoy).

So, let's talk vaccines and numbers.

First, vaccine efficacy = % reduction in disease incidence in a vaccinated group compared to an unvaccinated group under optimal conditions

Reference: who.int/influenza_vacc…

Also: herd immunity factor with no deaths

% of population infected to achieve herd immunity = (1-1/R0) x 100%

Where R0 = basic reproduction number, the no-interventions transmission ave. of the disease for every previously-infected case

Reference: academic.oup.com/jid/article/19…

For example w/ COVID-19 having R0 = 2 or 3, so say 2.5 (reference: imperial.ac.uk/media/imperial…),

(1 - 1/2.5) x 100% = 60% of the population should be infected without dying.

With a vaccine of 50% efficacy administered to 100% of the population, that is still not enough since

0.50 efficacy x 1.00 population = 0.50 or 50% of the population immune =/= 60% immune.

Buying a 50% efficacy vaccine isn't enough.

Especially damaging that even buying for the whole population, it is not sufficient.

Doubly damning is its expensive price.

Triply worse would be lack of published results and data on efficacy itself, especially on side effects.

So, as you can see, our math is

Efficacy x Population Vaccinated = Population Made Immune by Vaccination.

Now, let's try with Moderna, a more expensive vaccine, but much higher efficacy, at 94.5% (reference: google.com/amp/s/newsinfo…)

To achieve 60% herd immunity factor,

Vaccinated pop'n = 0.60 (immune)/ 0.945 (efficacy) = 0.6349 or 63.49%.

In this example, you have more leverage to reach herd immunity with manageable logistics especially for those who cannot be innoculated due to immunocompromised status.

Now there are cheaper vaccines still with even better efficacy than 50%, and it can be seen what makes total sense and why to surrender to 50% efficacy is NOT AN OPTION.