The pandemic has brought many misleading terms and ideas into everyone’s lives.Two particularly complicated concepts are the efficacy and effectiveness of vaccines. They are not the same. And as time passes and new variants such as omicron appear, one and the other also change. Melissa Hawkins is an epidemiologist and public health researcher at American University in Washington.
What do vaccines do?
A vaccine triggers the immune system to produce antibodies that remain in the body to fight future exposure to a virus.
The three vaccines currently approved for use in the United States, those from Pfizer-BioNTech, Moderna, and Johnson & Johnson, all showed success in clinical trials. What do the vaccines do?
A vaccine triggers the immune system to produce antibodies that remain in the body to fight future exposure to a virus.
The three vaccines currently approved for use in the United States, those from Pfizer-BioNTech, Moderna and Johnson & Johnson, all showed success in clinical trials.
What is the difference between efficacy and effectiveness of a vaccine?
All new vaccines must go through clinical trials in which researchers test the vaccines on thousands of people to see if they work and are safe.
Efficacy is the measure of how well a vaccine works in clinical trials. Researchers design trials to include two groups of people: those who receive the vaccine and those who receive a placebo. They calculate the efficacy of the vaccine by comparing how many cases of the disease occur in each group, vaccinated versus placebo.
Effectiveness, on the other hand, describes how well a vaccine performs in the real world. It is calculated in the same way, comparing diseases between vaccinated and unvaccinated people.
Efficacy and effectiveness are often close to each other, but they are not necessarily the same. How well the vaccines work will vary a bit from the trial results once millions of people are vaccinated.
Many factors influence the performance of a vaccine in the real world. New variants like delta and omicron can change things.
The number and age of people enrolled in trials are important. And the health of the people who receive the vaccine as well.
The proportion of the population that is inoculated can also influence its effectiveness.
Vaccines with moderate and even low efficacy may work very well at the population level.
Similarly, vaccines with high efficacy in clinical trials, such as coronavirus vaccines, may have less effectiveness and little impact if there is not a high acceptance of the vaccine in the population.
The distinction between efficacy and effectiveness is important: one describes the risk reduction achieved by vaccines in clinical trials, and the other describes how this may vary in populations with different exposures and levels of transmission.
Researchers can calculate both, but they can’t design a study that measures both simultaneously.
What about the omicron variant?
Preliminary data on omicron and vaccines are coming in quickly and reveal less vaccine effectiveness.
Best estimates suggest that vaccines are 30-40% effective in preventing infection and 70% effective in preventing serious illness.
A study conducted in Germany that has not been formally peer-reviewed found that blood antibodies collected from people fully vaccinated with Moderna and Pfizer showed reduced efficacy in neutralizing the omicron variant.
Other studies carried out in South Africa and England, which are also awaiting formal peer review, showed a significant decrease in the efficacy of antibodies against the omicron variant.
What do the first scientific studies say about the efficacy of vaccines against omicron
More disruptive infections are expected, with a decreased ability of the immune system to recognize omicron compared to other variants.