Were there enough people in the COVID vaccine clinical trials?

Yes, the vaccine clinical trials were designed to include enough people to make sure the vaccine is safe and works well. The authorized vaccines were tested in tens of thousands of people. And now tens of millions of Americans have received the vaccines. So if you get your vaccine now, you are not a guinea pig!

How many people are needed for a vaccine trial?

Epidemiologists design clinical trials to answer these questions: how will we know if a new vaccine works and is it safe? Vaccine trial designers decide what to measure (the primary outcome), how and how often to measure it, and how many people need to enroll in the trial to be confident in its results.

The best clinical trials are double-blind, placebo controlled trials. The double-blind part means that neither the researchers nor the study participants know who got the actual vaccine and who got the placebo. We use the placebo group to find out how often an outcome happens by chance. 

The number of people in a clinical trial is chosen to create enough “statistical power” to compare what happens in the vaccine group to what happens by chance in the placebo group. Exactly how many people you need depends on how common the primary outcome is, how well you want your new vaccine to work, and how confident you need to be in your results.

How many people participated in the COVID-19 vaccine clinical trials?

There were 43,448 people enrolled in the Pfizer and BioNTech vaccine trial. About half got the vaccine (21,720 people) and about half got a placebo (21,728 people). There were 30,418 people enrolled in the Moderna vaccine trial; about half got the vaccine and half got the placebo. Both trials were double-blind. 

According to the CDC, about 48.4 million doses of the Pfizer and Moderna vaccines have been administered in the U.S. as of February 12, 2021.

How many people normally enroll in vaccine clinical trials?

Every disease is unique, and so are all corresponding vaccine trials and vaccines. So it’s not all that helpful to compare the COVID vaccine trials to other vaccine trials. But we can look at recent flu vaccine clinical trials for a relative comparison. Researchers conducted 8 different randomized trials of trivalent inactivated flu vaccines from 1967 to 2011. The smallest trial had 728 participants. The largest trial had 7,652 participants. The COVID-19 vaccine trials were far, far larger than those flu vaccine trials.

So talk to your doctor about getting the vaccine when your turn comes!

Last update: February 10, 2021, 4:34 pm ET

Science review: BK, JAB