How Significant are Pfizer-Biontech's COVID-19 Vaccine Results?

 Pfizer has just announced that their COVID-19 mRNA vaccine has proven to be "90% effective" in preliminary stage 3 results. But what does this mean? 

Let's assume that Pfizer randomised the vaccine and placebo arms in a 1:1 ratio. this is not always the case- it's quite usual for trials to use a 2:1 ratio for randomisation. However, this is the most intuitive scenario, so let's use it.

From figures given, we understand that 94 subjects have developed COVID-19 across both arms of the trial so far.

Thus, there would be 22,000 subjects in each arm. Using Pfizer's own figures of 90% efficacy, it follows that 9 subjects in the vaccine arm and 85 subjects in the placebo arm developed COVID during the study.

(x/22000 divided by (94-x)/22000= 0.10, gives a value of x=9, approximately)

Plugging this into a 2x2 table:

                               Vaccine       Placebo

Disease                      9                   85

No Disease             21,991         21,915

Now you apply the chi-squared test, assuming that results would be acceptable with a confidence interval of 95%.

You thus get a p value of <0.00001.

It's very likely that the results are not due to chance, i.e. they show genuine protection against the virus.

However, there is a caveat. The Pfizer mRNA vaccine needs to be stored at -80 degree Celsius. It requires a cold chain. It must be used quickly after thawing. It might be a difficult ask particularly in countries such as India & Brazil.