What does it mean that COVID vaccine candidates have over 90% efficacy?
A major vaccine developer announced Monday that its coronavirus vaccine has an average efficacy of 70%, joining two others that reported an efficacy rate of over 90% in late-stage clinical trials.
Pfizer, Moderna and now the British-Swedish company AstraZeneca are in line to provide the country with three different COVID-19 vaccines if given government approval.
But what does vaccine efficacy mean?
Contrary to everyday language, it doesn’t carry the same definition as “effectiveness,” at least not in science.
“Vaccine efficacy” is measured in controlled clinical trials under “ideal conditions,” according to the Centers for Disease Control and Prevention. So, a vaccine efficacy of 90% means there was a 90% reduction “from the number of cases you would expect if [trial participants had] not been vaccinated.”
Drugmaker Pfizer said it measured its coronavirus vaccine’s efficacy by evaluating confirmed COVID-19 cases that occurred a week after participants’ second dose.
But data collection on new vaccines doesn’t stop after clinical trials. Researchers will continue to monitor how the COVID-19 vaccine prevents symptoms, severe disease and deaths over several years — in other words, the vaccine’s “effectiveness,” which can only be measured once the general population has been vaccinated.
This includes how well the vaccine performs in different groups of people, how long protection lasts and how well it can prevent infections from different virus strains that may evolve over time, wrote Zania Stamataki, a viral immunologist at the University of Birmingham in the U.K., in an article for The Conversation.
The U.S. Food and Drug Administration says in order for a COVID-19 vaccine to be effective, its efficacy “should be at least 50%” during clinical trials.
The World Health Organization also set a minimum efficacy of 50% for coronavirus vaccines, but it prefers “at least 70%.”
Stamataki said it’s unlikely a COVID-19 vaccine will be 90% effective once researchers see how it performs in the general public.
“Very few vaccines, aside from measles and chickenpox – are 90% effective. The flu vaccine is around 40%-60% effective, but it still saves millions of lives. And that’s something to celebrate,” she wrote.
No vaccine will ever be 100% effective because there will always be a small percentage of people who are either not protected by it or whose protection wanes over time, health experts with the The University of Auckland in New Zealand said. Some people also can’t get vaccinated due to certain medical conditions that suppress their immune systems.
