Why did the efficacy of China’s top vaccine drop from 78% to 50%?

January 13, 2021, 11:49 AM UTC

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Nearly one week ago, Brazilian officials sounded triumphant in announcing that the vaccine from Chinese drugmaker Sinovac was 78% effective in preventing COVID-19 infections.

“Today is the day of hope, the day of life,” João Doria, governor of Brazil’s São Paulo state, said at a press conference on Jan. 7.

But at a press conference on Tuesday, officials delivered a more sobering follow-up: Sinovac’s vaccine was, in fact, only 50.4% effective in preventing COVID-19 infections. The officials said that the lowered figure accounted for “very light” cases of COVID-19 among participants in the country’s Phase III trial of the vaccine that had been omitted in the earlier analysis.

Sinovac’s 50.4% efficacy rate in preventing COVID-19 infections is significantly lower than the 95% and 94% efficacy rates for Pfizer’s and Moderna’s vaccines, respectively. The lower figure reportedly came after Brazilian scientists pressured Sinovac’s Brazilian partner, the Butantan Institute, to release a fuller picture of its trial results.

Ricardo Palácios, medical director of the Butantan Institute, argued that the lower efficacy rate was evidence that Sinovac’s Brazil trials focused on higher risk groups than the Pfizer and Moderna trials and that the institute used more stringent standards. “We added all possible difficulties [to our trials],” he said. 

Neither Sinovac nor the Butantan Institute, however, has released full details of its trial data to the public, and Sinovac did not respond to Fortune’s request for comment on the vaccine’s new efficacy figure.

Experts tell Fortune that Sinovac’s lowered efficacy rate may not be as big a blow as it might appear, and that the vaccine may still prove a useful tool in helping bring an end to the global pandemic. At the same time, Tuesday’s announcement marks the latest in a series of inconsistent conclusions about the potency of Sinovac’s vaccine, which has now produced four different efficacy results across three continents. The discordant figures, combined with the dearth of publicly available data, may make it more difficult for the company to build public confidence in its doses.

Why did the rate drop?

A vaccine efficacy’s rate is calculated by comparing the number of COVID-19 infections among trial volunteers who received the vaccine versus the cases among volunteers in the placebo group. In the Brazil trial, 85 participants injected with Sinovac’s vaccine developed COVID-19 infections compared to 167 who developed infections in the placebo group, creating an efficacy rate of just above 50%.

At Tuesday’s press conference, Palácios said Sinovac’s relatively low efficacy rate of 50% was due to its more rigorous standard for what counts as an infection among trial volunteers. The Butantan Institute said it included six types of cases in its results: asymptomatic, very mild, mild, two levels of moderate, and severe. (The Brazilian authorities stressed that the public should focus on the vaccine’s 100% efficacy in protecting against severe cases of COVID-19.) Western vaccine makers generally included only mild, moderate, and severe categories, and companies like Pfizer are doing additional studies to see if the vaccine will prevent asymptomatic cases.

“People want to compare other studies, but that’s like comparing a person that runs a 1-kilometer race on a flat stretch and another doing it on a steep and obstacle-filled stretch,” Palácios said.

There is some evidence to back up Palácios’ claim that Brazil’s trial standards are more likely to catch mild cases of COVID-19 than trials conducted elsewhere, which would result in a lower efficacy rate.

In the U.S.’s Food and Drug Administration’s evaluation of Pfizer’s vaccine trials, the agency found that there were up to 1,594 suspected but unconfirmed cases of COVID-19. Because the infections were unconfirmed, they didn’t factor into the Pfizer vaccine’s efficacy rate.

“It’s true that other vaccine trials have not been able to confirm as many mild infections [as Sinovac],” says Ben Cowling, an epidemiologist at the University of Hong Kong. “The [vaccine efficacy] against mild disease could be lower than 90%-95% for Moderna and Pfizer.”

Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations, noted that Brazil’s trial was also largely made up of frontline health care workers. “They are more exposed to the virus and may explain the relatively low efficacy rate,” Huang said. Sinovac’s CEO Yin Weidong made that same argument when he addressed the lower efficacy rate at a press conference on Wednesday.


Cowling cautioned that Sinovac’s efficacy figure is still based on limited public data, and that a fuller data release is needed to fully assess the efficacy of Sinovac’s vaccine.

“I think we really need to see more comprehensive documentation before making any assessment of the vaccine performance” says Cowling. “What we have seen so far is pretty limited information.”

It’s not just the new efficacy figure in Brazil and the lack of public data that’s stirring confusion about how well Sinovac’s vaccine works.

Turkey has claimed that Sinovac’s vaccine is 91.25% effective in preventing COVID-19 infections, and Indonesia has said it’s 65% effective. (Both figures are based on interim trial data from the respective countries.)

The questions that remain about Sinovac’s vaccine hasn’t stopped the company from rolling it out. It has distributed jabs of the vaccine outside of clinical trials in China for months, and Indonesia used Sinovac’s vaccine on Wednesday to launch a massive campaign to vaccinate 181 million people. Alongside health ministers and Indonesia’s top Muslim cleric, Indonesia President Joko Widodo received the first dose of Sinovac’s vaccine on Wednesday in a televised ceremony from his presidential palace.

But there’s no guarantee that others will be as willing to take up the jab.

“[Transparency] is a concern,” says Huang. “Transparency about the data, even for the interim results, could still be very important to convince people that the results were indeed trustworthy.”

Even with the lingering unknowns, the vaccine may prove useful in inoculating millions of people who lack access to any alternative.

Sinovac has deals to supply lower-income countries like Malaysia, the Philippines, and Thailand, and its jabs, which require regular refrigeration, could serve as a lifeline for nations without the ultracold storage infrastructure needed to distribute the Pfizer and Moderna vaccines.

“Initially, I was surprised by how low that efficacy rate was, but after I gave some further thought on this the 50% actually wasn’t that bad,” says Huang. “It can still reduce the risk of COVID infection and be considered a game changer in the fighting of the current pandemic.”

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