• By DeeDee Stiepan

Phase 3 clinical trial of COVID-19 vaccine underway

August 6, 2020
a research workers marking test tubes in a Mayo Clinic vaccine research lab

As the race to develop a safe and effective vaccine to protect against COVID-19 continues, phase 3 trials of investigational vaccines are underway. In this Q&A, Dr. Gregory Poland, an infectious diseases expert and head of Mayo Clinic's Vaccine Research Group, explains the meaning of phase 3, who’s being tested and the hopeful outcomes.

Watch: Dr. Poland explains the meaning of phase 3, who's being tested and the hopeful outcomes.

Journalists: Sound bites with Dr. Gregory Poland are in the downloads at the end of the post. Please courtesy "Gregory Poland, M.D. / Vaccine Research Group / Mayo Clinic."

Q. What is a phase 3 vaccine trial?

A. A Phase 3 trial is really the last phase in the development of a vaccine before it goes through licensure. It is designed to measure efficacy and safety. After licensure, then we do so-called phase 4studies, which are really just attempts, once you're giving it now to hundreds of thousands of millions of people, to catch any side effects that you didn't in phase 3.

The phase 3 trials occurring now are designed to determine efficacy and safety. Efficacy meaning does it protect you against infection.

The Moderna vaccine trial has started. This trial will enroll 30,000 adults around the U.S., 10,000 of whom will get a saltwater placebo and 20,000 of whom will get the vaccine. In the same way, the Oxford vaccine trial will enroll 30,000 participants across Europe, Brazil and other locations.

Q. What will this phase help determine?

A. At the end of phase 3, what you'll know, hopefully, is did the vaccine prevent either acquiring the virus, or at least prevent disease or symptoms, and complications from that virus.

You'll know something about safety, but you'll actually only know about significant side effects that occur 1 in 5,000 times or more often. So, for example, a side effect that occurred 1 in 10,000 times won't be detected from this trial. That will await phase 4, when it's more widely used. So these are really important trials.

Q. Who are the vaccines being tested on? 

A. These trials will involve adults of various ages, but they won't yet be testing pregnant women or children, and that's a problem. As we've seen, children absolutely can transmit this virus and widely so. And for good reason, pregnant women are very concerned in regards to having a child during a pandemic like this. So at the current time, they're not included in these trials. Eventually, they will be.

The Moderna trial will occur in the U.S. and the Oxford trial outside of the U.S. For people who are interested, they can go to the coronaviruspreventionnetwork.org website and sign up to be a volunteer in the study, which of course is very helpful in trying to get this done. We can't develop a vaccine unless people are willing to enter these studies and take a vaccine, so it really is important.


Information in this post was accurate at the time of its posting. Due to the fluid nature of the COVID-19 pandemic, scientific understanding, along with guidelines and recommendations, may have changed since the original publication date

Check the Centers for Disease Control and Prevention website for additional updates on COVID-19. For more information and all your COVID-19 coverage, go to the Mayo Clinic News Network and mayoclinic.org.

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