Over the summer many experts were concerned that a possible COVID-19 vaccine might only be 50% effective, roughly similar to estimates of how effective the flu vaccine is. That is why the initial numbers reported by Pfizer-BioNTech and Moderna are particularly encouraging in the context of other vaccine efficacy rates.
Data from the CDC in this chart, which we've reproduced from Axios, compares the COVID-19 vaccines with those for other infectious diseases. If the 95% numbers turn out to be true and replicable then the COVID vaccines will rank among some of the most effective in modern medicine.
What does 95% effective mean?
Okay let's run some numbers using the Pfizer-BioNTech vaccine as an example. The trial involved 41,000 people, half were given the actual vaccine, half were given a placebo. At the end of the trial 170 cases of COVID-19 had been recorded in the total group. 162 of those were in the placebo group, and just 8 of them in the vaccine group. Hence the vaccinated group had 95% fewer infections = 95% effectiveness.
