Updated at 3:15 p.m. ET
Pfizer's experimental COVID-19 vaccine appears to be working. The vaccine was found to be more than 90% effective, according to clinical results released by the company Monday.
That news comes from an interim analysis of a study involving 43,538 volunteers, 42% of whom had "diverse backgrounds."
Each participant got two injections spaced 21 days apart. The analysis compared the number of cases of COVID-19 among the volunteers getting the vaccine with an approximately equal sized group of volunteers who got an injection of a liquid that didn't contain the vaccine.
In a news release from Pfizer and its partner BioNTech, the company said results from 94 evaluable cases of COVID-19 among study participants indicated the vaccine is more than 90% effective in preventing COVID-19. The Food and Drug Administration set a minimum effectiveness bar at 50%.
This is the first COVID-19 vaccine in development to have data showing that it exceeded that mark.
"I tend to be an optimist. I've been in vaccine development for over 35 years," said Dr. William Gruber, senior vice president of vaccine clinical research and development at Pfizer. "But this is extraordinary, and I think it speaks well not only for the potential of this vaccine but potentially other vaccines that are out there in development for COVID-19."
"This result is towards the high end of expectations," said an emailed comment from Shane Crotty, professor at the Center for Infectious Disease and Vaccine Research, La Jolla Institute for Immunology. "Greater than 90% efficacy at preventing disease, with 94 COVID-19 cases to evaluate, is an excellent outcome! It would be good to see more of the data, but those are very convincing numbers."
While promising, this analysis alone does not provide enough information about the vaccine for Pfizer to ask the FDA for permission to distribute it.
The agency has informed manufacturers that it wants a minimum of two months of follow-up data from at least half of the volunteers. The FDA says the reason for that requirement is that most dangerous side effects from a vaccine occur within two months of getting the final injection. Pfizer says that data won't be available until the third week in November.
The company didn't say how serious the COVID-19 cases were that occurred in the study, or whether any of them required hospitalization.
Natalie Dean, a biostatistician at University of Florida, says she's cautiously optimistic but waiting for more data.
"There remain many open questions, such as how well the vaccine is working across different age groups, how well it is preventing infection and severe disease," Dean wrote in an email to NPR. "But for the pre-specified primary endpoint of laboratory-confirmed symptomatic disease across the trial population, the numbers look very good."
The Pfizer vaccine trial is what's known as an event-driven study. It does not last for a specified period of time. Instead, the study will continue and won't be considered complete until it has recorded a total of 164 cases of COVID-19 among all the volunteers.
As described in its publicly released protocol, the company had planned to conduct four interim analyses, when a proscribed number of COVID-19 cases had occurred. The results released Monday are from the second one, the company having agreed with the FDA to forgo the first analysis.
AILSA CHANG, HOST:
And now some good news on vaccines. Pfizer and its partner BioNTech announced today some promising but preliminary results from a large study of their experimental coronavirus vaccine. The companies say their vaccine was more than 90% effective in preventing cases of COVID-19. And joining me now to explore the latest results and just how impressive they are is NPR science correspondent Joe Palca.
JOE PALCA, BYLINE: Hey, Ailsa.
CHANG: Hey. OK, so how impressive are these results?
PALCA: Oh, they're impressive (laughter).
CHANG: OK (laughter).
PALCA: They - I've been in communication with scientists all during the day. And I keep getting notes like, wow, this is - you know, we had hopes, but this is pretty remarkable. So yeah - very, very impressive.
CHANG: That's fantastic to hear. Well, tell us more about the study. Like, how big was this trial? And how did they determine whether the vaccine really worked?
PALCA: Well, this trial involved almost 44,000 volunteers. And the way it worked is everybody got two injections, but half of the people in the trial got an injection with just saltwater - placebo, it's called.
PALCA: And the other half got the vaccine. And then you sort of wait because you wait to see, well, how many people who got the vaccine get sick versus how many people got the placebo and got sick? Well, there are 94 cases of COVID in the study so far. And of those, more than 90% were in the group that didn't get the vaccine. So that suggests that your vaccine is working pretty well.
CHANG: That does sound promising. Well, I mean, vaccine studies have been criticized for leaving out certain populations like people of color or older adults. How diverse were the participants in this study? Do we know?
PALCA: Well, the company was sensitive to that - Pfizer. And they put out a list of their percentages by ethnic background, and I'll read them off to you. It's 6% Asian, 10% Black, 13% Hispanic-Latinx, 1% Native American and 45% 56 to 85 years old. So that's a pretty good spread. I should say those are the results from the study that was done in this country. There were other studies or other portions of the study that were going on in other countries. And they're slightly different, but it's still a pretty broad range.
CHANG: Well, why are we hearing about this news now, before the trial's complete? And why from the companies instead of, say, the Food and Drug Administration or a scientific journal?
PALCA: Yeah. Well, this is a little unusual in the sense that, I mean, normally drug companies don't talk much at all about the results from their studies until they're presented to the FDA, the Food and Drug Administration.
PALCA: But this is a different time. And people - they've been saying all along, look. As soon as we have definitive results either that the vaccine's working or it's not working, we're going to tell you. And they set a set of criteria, and they've been sticking to it as far as we can tell. I mean, they waited until they got X number of cases. Then they asked this independent body to review the data. And then the independent body looked at it and said, yes, it looks to us like you've got something there. And so they came out and said it. Nothing - it's unusual, but it's nothing out of the ordinary from what they said was going to be coming.
CHANG: OK. So what are we expecting to happen next at this point?
PALCA: Well, what happens next is they go to the Food and Drug Administration and ask for some sort of a permission to distribute this vaccine. It's not going to be a formal approval, but it'll probably be something called an emergency use authorization, which is allowed when we're in an emergency situation with a pandemic and we approve drugs or we - not approve but authorize drugs to be used a little bit sooner than they would normally. But the company says that it's going to take them at least three more weeks to get the kind of safety data that the FDA has asked for that they can present to the FDA. And then the FDA can make some sort of decision.
CHANG: All right. That is NPR science correspondent Joe Palca.
Thank you, Joe.
PALCA: You're welcome. Transcript provided by NPR, Copyright NPR.