What level of disease protection does the Covid 19 vaccine actually provide? Seems like a relatively simple question; it is not.
When Pfizer-BioNTech did its clinical safety-and-efficacy trials for its Covid 19 vaccine and then applied for “emergency use authorization” and recent FDA approval, they submitted data that demonstrated that their vaccine was “95% effective” against (relatively mild) Covid infection symptoms and 100% effective at preventing severe Covid symptoms that would require hospitalization. These clinical trial results from Pfizer (with similar trial results from several other pharmaceutical companies) were extremely robust and the vaccine was then approved for use by specific age groups in the U.S.
But what does “95% effective” really mean? The print and visual media–and even most health care professionals–continue to maintain (either out of ignorance or just a good-faith effort to get everyone vaccinated) that 95% effective means that in a cohort of, say, 100 participants all of which were vaccinated, (5%) would get Covid19 symptoms while 95 (95%) would not. So get vaccinated!
But, actually, a vaccine that is 95% “effective” does NOT mean that at all.
To see why this is so, we must understand that when Pfizer performed its clinical trials, it had to COMPARE the results from its fully vaccinated cohort group with a reasonably similar placebo group that had NOT been vaccinated at all. And it is THAT comparison that actually reveals the degree of protection provided by vaccination.
In the Pfizer clinical trials, there were 21,830 participants in the vaccine group and 8 developed symptoms associated with a Covid infection. There were 21,830 participants in the placebo group and 162 developed symptoms associated with a Covid infection. Thus the actual “risk of infection” in the vaccine group was 0.04 % while the risk of infection in the placebo group was 0.74%. The question then is: What level of protection (risk reduction) is actually provided by vaccination in comparison with the non-vaccinated placebo cohort?
To understand the answer to that question, we must compare the rates of infection WITH vaccination (in the first group) to the rates of infection WITHOUT vaccination (in the placebo group). This will provide us with the
so-called “absolute risk reduction” (ARR) associated with vaccination. So in our example, we must subtract the infection rate WITHOUT vaccines, which is 0.74%, from the infection rate WITH vaccines, which is 0.04%. This leaves us with 0.7% or the rate of Absolute Risk Reduction (ARR) associated with vaccination.
Now 0.7% of absolute risk reduction from infection is certainly a significant percentage and in a population where millions are vaccinated, it predicts (other things remaining the same) that there would be fewer cases of Covid disease. Nonetheless, the actual number of people “protected” from infection by vaccination in percentage terms (0.7%) is far less than, say, the number (95%) that is almost universally–and mistakenly–reported in the media as the so-called effective “protection” associated with the Pfizer vaccine.
Well, then, where does the number 95% come from and what does IT mean? We have already explained that in the clinical trials, the vaccine reduced the “infection risk” of Covid by 0.7%. This is the “absolute risk reduction (ARR) ” referred to above. On the other hand, a vaccine’s “efficacy” is the RELATIVE risk reduction (RRR) it provides vis-à-vis some non-vaccinated cohort group.
One can determine the “efficacy” or relative risk reduction (RRR) of the Pfizer vaccine by dividing the absolute risk reduction of its vaccine, which was 0.7%, by the infection risk in the placebo group, which was .74%. This calculation leaves us with the 95% “efficacy” percentage, the very number that appears in almost every media story concerning Covid and vaccine use. This degree of efficacy, by the way, is similar to the effectiveness of other drugs that the FDA has approved in the past, i.e., for mumps and measles as examples.
So what does all of this mean in practical terms? First, although the efficacy of the Pfizer vaccine is very high in preventing Covid symptoms, the absolute risk reduction associated with vaccination, as we have shown, is very low (.7%), less than one percent. Second, the efficacy of the vaccine in the clinical trials was estimated to last for about 6 months. Real world data now suggests that both the absolute and relative risk reduction associated with the vaccine may wane sooner than expected. Third, Pfizer’s vaccine was narrowly focused to combat Covid 19. It is unclear what level of protection is provided in a world where several new “variants” dominate new infections/cases.
Finally, in terms of public policy, it can be argued that vaccine protection “benefits” for various cohorts (age, previous medical history, etc.) would always have to be weighed against observed vaccine “costs” (such as mild-to-severe medical side-effects) and other non-medical concerns ( unintended economic consequences, restrictions on individual choice, etc.) before any individual or public health decisions concerning vaccination could be made.
