AILSA CHANG, HOST:

And joining us now with more is NPR science correspondent Joe Palca.

Hey, Joe.

JOE PALCA, BYLINE: Hello, Ailsa.

CHANG: Hi. So as we just heard, U.K. health secretary is calling today V Day in the U.K. Can you just put into context how important today is not just in the U.K., but globally?

PALCA: Well, I think they're entitled to a little bit of crowing. This is symbolically big, but it's a start. I mean, not everyone in the country was vaccinated today. That would be, you know, the big invasion kind of V Day. But, you know, this is the first Western country to begin a major vaccine campaign, so it's important.

CHANG: Right. Well, OK, this Pfizer-BioNTech vaccine that's being given in the U.K. right now - it isn't being distributed in the U.S. yet. When can we expect that to happen?

PALCA: Well, it's probably soon, but it's a little hard to say exactly when. On Thursday this week, an independent committee of experts will meet to advise the Food and Drug Administration whether to give the vaccine emergency use authorization so it could be distributed in the United States. And in advance of that meeting, the FDA today released its analysis of the Pfizer and BioNTech application for that authorization.

CHANG: And tell us what the FDA had to say about that today, about the data the companies have presented.

PALCA: Yeah, they were pretty positive. The analysis was based on a study of the vaccine companies - that the vaccine companies conducted in some 44,000 volunteers. Half got two shots of the vaccine. Half got two shots of a placebo. And here I - quoting from the analysis, based on the totality of scientific evidence available, it is reasonable to believe that the vaccine may be effective in preventing COVID-19 in individuals 16 years of age and older. And the known and potential benefits of Pfizer-BioNTech COVID-19 vaccine outweigh its known and potential risks for use in individuals 16 years of age and older.

Now, that's regulatory agency speak. But I asked Gregory Poland for his take on the FDA's analysis of the data. And Poland is a vaccine developer at the Mayo Clinic and has been an adviser to the FDA on vaccines in the past. I reached him just after he finished reviewing the document.

GREGORY POLAND: I was trembling. I was trembling. To get a grand slam like this is unbelievable. In my wildest imagination, I would not have dared to hope for data of this clarity and quality.

CHANG: Wow, grand slam. I mean, he makes it sound really promising, but I understand there were side effects, right? What were these side effects?

PALCA: Yeah, but - yeah. And they weren't trivial, but they weren't serious. I mean, people had pain in the injection site. They had fatigue. They had headache. They had muscle pain. They had chills. They had joint pain. They had fever. But they all resolved. But having these side effects - there is a real problem, as Gregory Poland pointed out.

POLAND: The problem is that when you have those symptoms, we have learned to associate it with infection, getting sick, something wrong. And it's not. It's an indication that your immune system is robustly responding to this.

PALCA: So it's the vaccine that is causing this. And it's getting your immune system primed and ready so that if the real virus ever comes along, it'll be able to fight it off.

CHANG: OK. So what happens next with the FDA analysis?

PALCA: Well, as I mentioned, the VRBPAC, the Vaccines and Related Biological Products Advisory Committee, meets on Thursday. And they'll go over the data to see if they agree with FDA's analysis. They'll also be asked to say what kinds of studies should be conducted going forward to keep out - look out for potentially rare side effects. And one issue that'll probably come up is what to do about the people who got the placebo in the trial. The companies want to give them the vaccine, but the FDA's not totally inclined to do that. And Greg Poland agrees it's not - it's important not to unblind the study, meaning let people know whether they got the placebo or the vaccine.

POLAND: If that happens, we will not know about long-term efficacy. We will not know about longer-term safety. We won't be able to answer some secondary questions.

CHANG: And finally, I understand that researchers published some data today on another vaccine. Can you tell us about that one?

PALCA: Yeah, this is a vaccine made by a collaboration between AstraZeneca and Oxford - University of Oxford. And it's a different kind of vaccine from the Pfizer vaccine and the Moderna vaccine, which is very similar to Pfizer's. The results from this study - these studies were that the vaccine was 70% effective, but there were some strange things about the study. There was a low dose/high dose combination that was more effective than a high dose/high dose combination. But people are very eager for this to work because it's cheaper, it's easier to make and it doesn't require special freezers. So it's easier to use and distribute in countries of low resource.

CHANG: All right. That's NPR's Joe Palca.

Thank you, Joe.

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