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How Volunteers And Scientists Help Determine If A Vaccine Works


There are now three potential COVID-19 vaccines undergoing wide-scale testing in the U.S. Two more potential vaccines are also about to hit that phase. So how do we know whether these potential vaccines will be effective, and how soon will we know? NPR's Joe Palca is here to help answer those questions. Hey, Joe.

JOE PALCA, BYLINE: Hey, there.

KING: All right. Let's start with the basics. So these potential vaccines will be tested in 30,000 people each. Is that a lot of people?

PALCA: (Laughter) It sounds like a lot to me. Yes, it's going to be about 15,000 in each group. They divide them in half. Half will get the vaccine; half will get a placebo, just an inert shot. And why 30,000? Well, it turns out that you have to test the vaccine in a lot of people because you don't know who's going to be exposed to the virus. And so you test in a lot of places around the country hoping that that's where there will be virus circulating. I mean, think about it. A couple months ago, you would've said, oh, test in New York. There's...

KING: Yeah.

PALCA: ...A lot of cases there. But if you did that now, there are very few cases in New York. So it'd take a long time for people to be exposed. I spoke with Holly Janes about this. She's a biostatistician at Fred Hutchinson Cancer Center in Seattle. And she told me, early on, it was decided to enroll a lot of people.

HOLLY JANES: Primarily due to the uncertainty as to where those infections and disease endpoints will happen.

PALCA: So - but basically, if I understand you correctly, what you're saying is if you were in a situation where people were left and right all the time getting sick with COVID, if you wanted to test a vaccine under those circumstances, you wouldn't need so many people.

JANES: That's exactly right. Right. If we knew exactly where that would happen, we could do a smaller trial.

KING: But for now, they're sticking with a larger trial - 30,000 people. Half of them get the real vaccine; half of them get an injection with basically nothing in it. What happens then?

PALCA: You wait. You wait for what are called primary endpoints.

JANES: These trials will use as a primary endpoint laboratory-confirmed COVID disease.

PALCA: That was Holly Janes again. She says so, OK, if you get sick, you go and get tested to make sure that it was COVID and not something else. And you're looking to see if there are at least 50% fewer cases in the vaccinated group compared to the control placebo group.

KING: And how long does it take to figure that out?

PALCA: Well, that's not clear because Holly Janes says these are the kinds of trials called an event-driven trial. And in this case, an event is a case of COVID-19.

JANES: An event-driven trial means that the primary analysis of the trial happens when you get enough events. And we don't know how long that's going to take.

PALCA: In other words, when enough people get COVID, they - so they can statistically meaningfully make a comparison between the vaccine group and the placebo group, that's when you stop the trial. And Janes says that the number will be around 150 cases of COVID-19 among the entire population of the people in the trial.

KING: OK. That part makes sense. But the people giving the injection don't know what's in the syringe, and the people getting the injection don't know what's in the syringe. So when you do get those cases of the virus, how do you know whether these people were in the placebo group or the vaccinated group?

PALCA: Right. This is what's called a double-blind, placebo-controlled trial. And the people who know are a group of people called the Data and Safety Monitoring Board. And these are completely independent people. They have nothing to do with the trial. Their only job is to look at the results from time to time, and they're looking to mostly make sure that there are no bad side effects, that people aren't getting sick from the vaccine. But they're also looking to see if there's a clear indication that the vaccinated group are getting fewer cases of COVID than the control group.

KING: And when could we have an answer by?

PALCA: Well, the president has been hinting that there could be an answer, you know, even as soon as the end of October. But Moncef Slaoui, who's the chief scientific adviser to the administration's Operation Warp Speed, appeared to pour cold water on that in an interview he did last night with All Things Considered.


MONCEF SLAOUI: There is a very, very low chance that the trials that are running as we speak could read before the end of October.

KING: What's the drawback to putting out a vaccine too soon? What does too soon mean?

PALCA: Well, too soon means if you're not sure - if the vaccine doesn't actually work, then people could take it and get sick and die if they were told it was safe and effective and it's not. And if it wasn't safe, then perfectly healthy people would put their health at risk by taking the vaccine. And if the vaccine is perceived as a flop by the public, it will undermine confidence in the government 'cause they said it was safe and effective. And confidence in the government is something that's in short supply at the moment.

KING: Yeah, fair enough. NPR's Joe Palca. Joe, thanks so much.

PALCA: You're welcome. Transcript provided by NPR, Copyright NPR.

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.