DAVID GREENE, HOST:
According to the World Health Organization, there are now 16 vaccine candidates for COVID-19 that are being tested in human volunteers. Most of them are still in the earliest stages, making sure the experimental vaccine doesn't cause any worrisome side effects. Some are further along, looking for initial signs that the vaccine might actually prevent someone from getting sick. Let's talk about the whole vaccine landscape with NPR science correspondent Joe Palca. Hi, Joe.
JOE PALCA, BYLINE: Morning, David.
GREENE: So these 16 different vaccine candidates - I mean, are they the same or they differ in important ways?
PALCA: Well, they differ in various important ways. But let's start by talking a little bit about what a vaccine does. So a vaccine takes something that looks like the virus - in this case, the coronavirus - and it injects it into someone, and it says to the person's immune system, hey, this is what this virus looks like. But I'm not actually showing you the virus; I'm showing you something that looks like the virus. And the analogy I've been using is, it's like showing someone a picture and saying, if this person shows up at your door, don't let them in.
GREENE: Oh, interesting.
PALCA: So here's the thing (laughter) - to pursue the analogy a little further, vaccine manufacturers have to do the equivalent of saying, OK, do you go door to door and show people the picture? Do you put them on highway billboards? Can you use them on pop-up ads in the computer? So the designers have a lot of choices to make about what kind of picture to use and how best to present it to the immune system.
GREENE: So, I mean, scientifically, what are some of the different ways they try to accomplish that?
PALCA: Well, the immune system sees this protein on the surface of the virus, and so most - if not all - vaccines are showing the immune system this virus surface protein called the spike protein. Now, some work by just injecting a bolus of this protein into somebody and say, there it is. Some work by taking the entire coronavirus and killing it, inactivating it and then injecting the inactivated vaccine - that's a little bit like what the Salk polio vaccine was like. Some employ what's called a viral vector to get the genetic instructions for making the spike protein. And some just inject their RNA directly into somebody and get that to make the spike protein.
GREENE: So are we at a point now where we can say which of these approaches is looking most promising and is furthest along?
PALCA: Well, what happens is there's, first, safety, then there's the - you're trying to see if it generates an immune response, and then you actually see if it's effective, which means if someone gets exposed, will they be protected? And there's a vaccine made by the University of Oxford, cooperating with AstraZeneca, that's using this viral vector, and they're actually into this stage of testing where they see, OK, here's the vaccine group and here's a nonvaccinated group - or not vaccinated with the COVID vaccine. Who gets sick? And so if the vaccinated group gets sick at a lower rate, then you know you've got a vaccine that appears to work.
There's also a Chinese biotech company that's got a vaccine that's getting close to that stage. And then there's this American company called Moderna, and they use this - injecting the genetic material called - it's called an RNA vaccine. And those three are kind of in the lead. But there are several more that are really close behind, and there's a good chance they'll be getting to their own efficacy testing later this summer.
GREENE: And I guess the big question everyone wants to know is when could we actually know that one of these really works?
PALCA: Well, the interesting thing is part of the answer depends on how much of the virus is circulating because if a lot of people are being exposed and the vaccine's working, then you'll see a lot of people being protected. And if you don't have a lot of people exposed, then it's hard to know if your vaccine is working or not. But that could be by the end of the year - we could know.
GREENE: That would be good news. NPR's Joe Palca. Joe, thanks so much.
PALCA: You're welcome.
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