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Why Coronavirus Vaccine Trials Need Large Numbers Of Volunteers


More than 200,000 people in the U.S. have now died from COVID-19, and scientists around the world are racing to develop a vaccine to prevent the disease. So far, they've enrolled more than a hundred thousand volunteers to test the leading vaccine candidates. NPR science correspondent Joe Palca has been looking into how we will know whether a vaccine is truly effective and when we might have that answer.

Hi, Joe.


PFEIFFER: First off, would you recap for us why we need so many people in these trials?

PALCA: Well, one of the reasons is you want to be able to look for rare side effects. I mean, the initial trials involve tens or maybe hundreds of people, and there are sometimes side effects that only show up when you look at a lot of people. But the other thing is you need to make sure people get exposed. And this is where things get really interesting because researchers can't go out and tell people, yes, now if we've given you a shot, go see if you can get exposed to the virus. That would be unethical.

I spoke with Ruth Karron, director of the Johns Hopkins Vaccine Initiative, and she says volunteers will be told to do all the things that will keep them healthy - hand-washing, wearing a mask, social distancing.

RUTH KARRON: Some people will not be able to adhere to that, and some people will have exposure just by the nature of the kind of work they do. For example, it could be if they're health care workers or if they're other kinds of frontline workers.

PALCA: And also, they need a lot of people because the trials will have two groups - people who actually get the vaccine and people who just get an injection with an inert liquid, like saline.

PFEIFFER: And since there are those two different groups, how do you know that the vaccine is responsible for people not getting ill? I mean, maybe more people in the vaccinated group just never left their homes and so had no contact with anyone who could expose them to the virus.

PALCA: Exactly. Well, that's a possibility. But there are two good reasons why that doesn't happen. First of all, there's something called double-blind, which I'm sure you've heard of. It means...


PALCA: ...When you get jabbed in this study, you don't know what you're getting jabbed with, and the person doing the jabbing doesn't know what he or she is giving you. So there's no basis for changing your behavior based on whether or not you think you got the vaccine or didn't think you got the vaccine.

But the other thing is something called randomization. And yes. OK. They'll be some people be hermits. And they'll some people who go out with a devil-may-care attitude, and they'll expose themselves. But the idea with randomization is those people will be equally distributed between the vaccine group and the placebo group. And so you get the same numbers doing the same sorts of things, and so you can make a meaningful comparison between the two groups.

PFEIFFER: And then how do researchers know if anyone's actually being exposed to the virus?

PALCA: Well, I mean, the truth is you don't. In an individual case, you're making a bet that, with a lot of people being out in the world, some people will be exposed. And you want to test the vaccine in a place where there are people - where the virus is circulating.

But the other thing is you have to wait until you get the - a certain number of results. They call it events in the trial lingo - the number of people getting sick. And this is why big numbers are needed - because if you get the first 10 people and they all turn out to be in the control, in the placebo group, it's - oh, the vaccine's working. That's fantastic. But it may be a statistical fluke. And the next 10 people might be all in the vaccinated group who get sick. So you really have to wait until you get a certain number of cases. And in this - in these studies, they think that number is around 150 cases of disease. And then they'll look to see, well, how do those match out between the control group, the placebo group and the people who actually get vaccinated?

PFEIFFER: Joe, is there any way to show that a vaccine works without involving so many people?

PALCA: Well, there is a way. You can do what's called a challenge trial where you actually take the virus - after you vaccinated somebody, you take the virus and put it into their nose. But this has got ethical issues with it because you don't have a treatment. And if they get sick, you could actually be killing people in your trial. So you're kind of betting that the vaccine will work, and some people feel that's not really ethical to do.

PFEIFFER: That is NPR science correspondent Joe Palca.

Joe, thank you.

PALCA: You're welcome.


Joe Palca is a science correspondent for NPR. Since joining NPR in 1992, Palca has covered a range of science topics — everything from biomedical research to astronomy. He is currently focused on the eponymous series, "Joe's Big Idea." Stories in the series explore the minds and motivations of scientists and inventors. Palca is also the founder of NPR Scicommers – A science communication collective.