Vaccine efficacy what does it mean?

Wednesday, September 7, 2022

Letter to the editor, Some people are confused about why some people who have been vaccinated still get Covid- 19. The answer is simple. There has never been a vaccine that is 100% effective.

Each fall, doctors encourage us to get a flu vaccine. We need a new vaccine yearly for several reasons: viruses undergo mutations that can result in different viral strains or variants; there are over sixty different influenza variants worldwide; manufacturing a vaccine takes months & is based on predictions of which flu strains/variants will be prevalent in the coming year. We get a booster shot geared to a different combination of virus variants each year. Flu vaccines typically range from 40-60% effective. Fortunately a quadrivalent flu vaccine is now used, which protects against four separate influenza variants.

Like many others, I have been confused about what vaccine efficacy and vaccine effectiveness mean. Vaccine efficacy is determined during a clinical trial in which a control group of individuals receives the vaccine and a separate group receives a placebo (an injection with no vaccine). Researchers follow-up over time with each individual to discover whether or not they become ill. Clinical trials are completed before a new vaccine is approved for use.

For example, when the first shingles vaccine became available, it had 60% efficacy. Those of us who have had family members who suffered through shingles knew how important it was to get that vaccine, even with a low rate of efficacy.

Efficacy tells us that, in the case of the shingles shot, there was a 60% reduction in cases of shingles in the vaccinated control group com- pared to the unvaccinated group in the clinical trial. Unfortunately, this is a difficult concept to understand and has not been explained well to the general public.

In 2017, a new shingles shot became available that has a reported 90% efficacy rate. Those of us who understood what an astounding difference that was, when compared to the previous shingles shot, raced to our doctors or pharmacists to get the new two-dose shingles vaccine.

The vaccine efficacy rate is predictive of how the vaccine will perform in the general population, but once a vaccine is put into use, additional research studies are conducted to verify how well the vaccine performs in the general public. Those studies tell us how effective the vaccine is in the real world. And they are never 100% effective.

Dr. Delena Norris-Tull Dillon

Editors note: This is the first in a three-part series.