OMICRON: How effective are vaccines against omicron? An epidemiologist answers 6 questions

The pandemic has brought many tricky terms and ideas from epidemiology into everyone’s lives. Two particularly complicated concepts are vaccine efficacy and effectiveness. These are not the same thing. And as time goes on and new variants like omicron emerge, they are changing, too. Melissa Hawkins is an epidemiologist and public health researcher at American University. She explains the way researchers calculate how well a vaccine prevents disease, what influences these numbers and how omicron is changing things.

1. What do vaccines do?

vaccine activates the immune system to produce antibodies that remain in your body to fight against exposure to a virus in the future. All three vaccines currently approved for use in the U.S. – the Pfizer-BioNTech, Moderna and Johnson & Johnson vaccines – showed impressive success in clinical trials.

2. What is the difference between vaccine efficacy and effectiveness?

All new vaccines must undergo clinical trials in which researchers test the vaccines on thousands of people to examine how well they work and whether they are safe.

Efficacy is the measure of how well a vaccine works in clinical trials. Researchers design the trials to include two groups of people: those who receive the vaccine and those who receive a placebo. They calculate the vaccine’s efficacy by comparing how many cases of the illness 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 the same way, by comparing illness among vaccinated and unvaccinated people.

Efficacy and effectiveness are usually close to each other but won’t necessarily be the same. How the vaccines work will vary a bit from the trial results once millions of people are getting vaccinated.

Many factors influence how a vaccine performs in the real world. New variants like delta and omicron may change things. The number and age of people enrolled in the trials matter. And the health of those receiving the vaccine is also important.

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