In a reply to a comment last week, I goofed in explaining the efficacy calculation. I have a feeling I’m not the only one who, if asked to describe it on the spur of the moment, would err. So today I’d like to correct that. I’m relying primarily on two sources: Principles of Epidemiology in Public Health and This Week in Virology, #695.
Dr. Daniel Griffin makes it clear that the Phase 3 trials for Pfizer and Moderna have so far measured disease efficacy. According to the CDC article, “Vaccine efficacy/effectiveness is interpreted as the proportionate reduction in disease among the vaccinated group. So a VE of 90% indicates a 90% reduction in disease occurrence among the vaccinated group, or a 90% reduction from the number of cases you would expect if they have not been vaccinated.”
Let’s see how this applies to the results from Pfizer .
In the placebo group, out of 22,000 people, 132 developed disease, i.e. symptomatic COVID. In the group that received the actual vaccine, only 8 people got symptomatic COVID.
The formula for calculating efficacy from the CDC:
So this would result in: ( 124 – 8 ) / 124 = 93.5%
And here’s where the difference between disease and infection is so important. The efficacy for both Pfizer and Moderna vaccines is based on disease, not infection. That means that in both the placebo group and the vaccinated group, there is an unknown number of people who were infected with COVID, but never got the disease, i.e., symptoms.
And that’s why, once you get vaccinated, you still have to wear a mask and socially distance until we achieve herd immunity. It makes sense that the focus would be on disease efficacy instead of infection elimination, given the current state of our hospital and ICU capacity and the alarming increase in deaths, as well as severe time constraints. It may take 6 months to a year before we know if the vaccines prevent infection as well.
A note about herd immunity. This doesn’t mean the virus is eradicated. It simply means enough people have resistance to the virus that it can’t spread. Even if the vaccines don’t eliminate the virus in your body, it is still possible for us to achieve herd immunity.
One last thing on the vaccines and allergies. Dr. Griffin states that the warnings about allergies are for people who have had adverse reactions to vaccinations, not people who have hay fever, peanut allergies, etc.