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Many countries around the world are betting on a vaccine from China to help them stop the coronavirus. On Sunday, for example, Brazil gave emergency use authorization to this vaccine made by the Chinese pharmaceutical company Sinovac. Countries are embracing the Chinese vaccine despite conflicting reports about how well it works. NPR health correspondent Maria Godoy reports.
MARIA GODOY, BYLINE: Nearly two weeks ago, researchers running a late-stage trial for Sinovac's vaccine in Brazil announced it was 78% effective against COVID, which sounds pretty good. But then last week, they revised those numbers, saying the vaccine prevented disease only about 50% of the time. So what's the truth? Natalie Dean, a biostatistician at the University of Florida, says it all depends on how you define efficacy.
NATALIE DEAN: So when we talk about vaccine efficacy, often we think about a single number. But actually, there are a lot of different types of efficacy, and you could think about it as a spectrum.
GODOY: Dean says vaccines typically work best at preventing severe disease. That's what the Brazil trial found - 78% of the time, the Sinovac vaccine protected people against moderate or severe disease or even mild disease that needed some medical assistance. But when the researchers included what they called very mild symptoms that needed no medical attention, the vaccine's effectiveness dropped to 50%.
DEAN: As we tend to include milder and milder cases, it's natural to see a bit of a drop in vaccine efficacy.
GODOY: Sinovac is conducting trials in various countries, and they haven't released much of the data. John Moore, a vaccine researcher with Weill Cornell Medicine, says that's made it hard for outside scientists to know exactly what's going on.
JOHN MOORE: It's science by press release. The Chinese are being, well, characteristically less than transparent.
GODOY: Even so, from the data that is available, Moore says it's clear that the Sinovac vaccine is less effective than those from Moderna and Pfizer. Those vaccines, on the whole, protect against disease around 95% of the time. Moore says that's not surprising because previous data had showed that the vaccines from Moderna and Pfizer, both of which use a new technology called mRNA, triggered a stronger immune response than the Sinovac vaccine.
MOORE: Most people in the field believe that the antibody response is the correlative protection and that, you know, the strength of the antibody responses matters.
GODOY: Natalie Dean says the less effective a vaccine is at preventing disease, the harder it is to pinpoint its efficacy.
DEAN: When the vaccine is closer to 50, 60%, that range of uncertainty can be a lot bigger.
GODOY: That may help explain another head-scratcher. Different clinical trials in different countries have reported starkly different efficacy rates for the Sinovac vaccine, from 50% in Brazil to 65% in Indonesia to a stunning 91% in a smaller study in Turkey.
DEAN: You need a lot more data to distinguish between different levels of efficacy, so it might also just be kind of statistical noise to a degree.
GODOY: But even if the Sinovac vaccine is only around 50% protective, that's still substantial. It's better than the flu vaccine some years, and it does meet the minimum threshold for emergency use authorization set by the World Health Organization. That's not nothing.
DENISE GARRETT: It's much better than nothing.
GODOY: Dr. Denise Garrett is an epidemiologist with the Sabin Vaccine Institute in Washington, D.C. She says the data show the Sinovac vaccine protects against severe cases of COVID, and that could have a big impact in places like her native Brazil.
GARRETT: The health care in Brazil is about to collapse in many cities. The situation is very critical. And having a vaccine that will prevent people from being hospitalized - that will be a great impact for us.
GODOY: Maria Godoy, NPR News.
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