What Is the Difference Between a Vaccine Clinical Trial and a Vaccine Challenge Study?

Dr. Paul Offit explains the difference between vaccine clinical trials and vaccine challenge studies, using COVID-19 vaccine development as an example.


Paul Offit, MD: Hi, my name is Paul Offit from the Vaccine Education Center at the Children's Hospital of Philadelphia. We're in the midst of doing these COVID-19 vaccine trials. Now, the way that these trials normally work is that you inoculate tens of thousands of people with a vaccine, you give tens of thousands of people just a placebo, usually saltwater, and then you send them out into the world. And, you see who gets sick and who doesn't, and it takes a while for that to happen.

Some people argue, why not just speed up the process? Why not essentially inoculate people who've gotten the vaccine with the virus and see whether or not they're protected. There's a couple of problems with that. First of all, although it probably would speed up the process somewhat, it is artificial. In other words, typically, when you're infected with the virus in the real world, you're infected because someone coughs or sneezes or talks in your direction, and then you're infected with a variety of different levels of virus. Some people are infected with just a little bit of virus; some people with more virus; some people with a lot of virus. And, usually those who were infected with a greater quantity of virus, a greater amount of virus, are more likely to get sicker than those who were infected with a small amount of virus. When you do these human challenge studies to try and speed it up, usually only a single dose is picked, so it doesn't really mimic the natural situation. It's also not natural in the sense that you're really just inoculating the person with that virus rather than having them be exposed to small droplets from talking or sneezing or coughing. That's one of the problems.

The second problem is that you really can't do those kinds of studies in people over 65 years of age. You can’t inoculate somebody who's older, who’s much more likely to die from this virus than someone who's younger, with that SARS-CoV-2 virus when you know that you don't have a rescue drug; you know that you don't have a drug that you can say with certainty would save that person's life.

So, although thousands of people, frankly, tens of thousands of people have offered to be part of these human challenge trial experiments, for the most part, they haven't gotten off the ground because, really, the virus is so contagious, and there are so many people now that are being infected that you really can do the studies you need to do fairly quickly just by putting people in the natural situation.

Thank you.

Related Centers and Programs: Vaccine Education Center

Last Reviewed on Dec 02, 2020