Many countries are betting on a vaccine from China to help them stop the coronavirus — although questions are surfacing about whether it's a smart bet.
The vaccine is made by the Chinese pharmaceutical company Sinovac. This week, Brazil began rolling out its national immunization program after granting emergency use authorization for the vaccine over the weekend. Indonesia and Turkey started their mass vaccinations with Sinovac's vaccine, called CoronaVac, last week.
But there are conflicting reports about just how well the vaccine works.
Nearly two weeks ago, researchers running a late-stage trial for Sinovac's vaccine in Brazil announced it was 78% effective against COVID-19, which sounds fairly impressive. Then, last week, the researchers 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.
"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," explains Dean, who specializes in infectious diseases and vaccine study design.
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 — and 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%.
"As we tend to include milder and milder cases, it's natural to see a bit of a drop in vaccine efficacy," Dean says.
But there's another issue. Even though Sinovac is conducting trials for its vaccines in various countries, the company hasn't released much of the data. John Moore, a vaccine researcher and immunologist with Weill Cornell Medicine, says that lack of openness has made it hard for scientists not involved in the Sinovac vaccine trials to know exactly what's going on.
"It's science by press release," Moore says. "The Chinese are being, well, characteristically less than transparent."
Even so, from the data that are available, Moore says it's clear that the Sinovac vaccine is not as effective as those from Moderna and Pfizer. Their vaccines protected against developing symptomatic COVID illness, from mild to severe, 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 antibody response than the Sinovac vaccine, which uses an inactivated version of SARS-CoV-2 to induce an immune response.
"Most people in the field believe that the antibody response" correlates with the protection a vaccine provides, Moore says, "and the strength of the antibody response matters."
Dean says the less effective a vaccine is at preventing disease, the harder it is to pinpoint its efficacy. "When the vaccine is closer to 50, 60% [effective], that range of uncertainty can be a lot bigger."
That may partly 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 Turkey, based on a smaller study.
"You need a lot more data to distinguish between different levels of efficacy," Dean says. "So it might also be just kind of statistical noise to a degree." Without seeing the full data from all the Sinovac vaccine trials, it's impossible to know for sure, Dean notes.
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 listing set by the World Health Organization.
"It's much better than nothing," says Dr. Denise Garrett, an epidemiologist with the Sabin Vaccine Institute in Washington D.C.
Garrett says the data from Brazil does indicate that the vaccine protects against severe cases of COVID — and that could have a big impact in places like her native Brazil.
"The health care in Brazil is about to collapse in many cities," Garrett says. "The situation is very critical. And having a vaccine that will prevent people from being hospitalized, that will be will be a great impact for us."
Dr. Paul Offit, director of the Vaccine Education Center and a member of the Food and Drug Administration's vaccine advisory board, agrees: "To me, the goal of this vaccine is to keep you out of the hospital and keep you out of the morgue."
The downside of a vaccine with just 50% efficacy is that you will have to vaccinate just about the whole population to reach herd immunity, Garrett notes.
But the Sinovac vaccine does offer another advantage: The company has the ability to manufacture immense quantities of the vaccine.
With vaccines from Pifzer and Moderna in short supply, for countries like Indonesia, where COVID cases are surging, the calculus may well be to bet on the vaccine they can access right now, says Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations. "The China-made vaccine is the one that is readily available and that appears to be effective. So why not?"