“Vaccine efficacy” refers to how well a vaccine works in clinical trials whereas “vaccine effectiveness” refers to how well a vaccine works in everyday “real life” in the general population (or a specific population being studied).
In both cases, the number provided refers to the relative reduction in infection or disease for the vaccinated arm compared to the unvaccinated arm. A perfect vaccine effective of 1, or 100%, means there’s a 100% reduction of infection/disease in the vaccinated population. In other words, no one gets the disease. Read more about the differences on efficacy vs effectiveness in the medical studies definition here.
Deeper dive
A common misconception about vaccine effectiveness assumes sickness in the population who are not immune to the disease, but the unprotected population is simply susceptible to the disease, not necessarily definitely infected. For example, if a vaccine is 95% effective, that does not mean that 5% of people who receive the vaccine will definitely get sick. It means that 5% of people who get the vaccine are susceptible to getting the disease, and how many actually get it will depend on their exposure.
As another example, assume a vaccine is 80% effective. That means if you expose two separate populations to the pathogen, one vaccinated and one not vaccinated, then 80% fewer of the vaccinated group will get sick than the unvaccinated group. There will still be people who get sick in the vaccinated group, and there will be people in the unvaccinated group that DO NOT get sick, but overall, the incidence in the vaccinated group will be 80% lower than the incidence in the unvaccinated group.
In a large population where the majority of people are vaccinated, it’s easy for more vaccinated than unvaccinated people to get sick with the disease. That does not mean that vaccinated people are more likely to contract the disease. It’s simply that when you compare a small proportion of a large number to a large proportion of a small number, that “small proportion” can end up being numerically larger than the “large proportion.”
Here’s an example with numbers: Assume that you have a population of 5.2 million people:
- Five million people are vaccinated, and 200,000 people are unvaccinated.
- Let’s assume the vaccine is approximately 92% effective. (If we want to get specific with the math, it will be 91.7% effective.)
- If 1.5% of 1 million vaccinated people get the disease, then that’s 75,000 people who are sick.
- If 18% of 200,000 unvaccinated people get the disease, then that’s 36,000 people who are sick.
In this example, more than twice as many vaccinated people got sick (75,000) than unvaccinated people who got sick (36,000), which might make it seem, based only on the numbers, like the vaccine was not effective. But if we look at the percentages of people who got sick in each group, we see that 18% is much higher than 1.5%. In fact, 1.5% is 91.7% lower than 18%. (The math: 18 x 0.917 = 16.5, and 18 – 16.5 – 1.5.) The vaccine was about 92% effective.