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Why has the effectiveness of the main Chinese vaccine increased from 78% to 50%?

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Nearly a week ago, Brazilian officials looked triumphant when they announced that the vaccine from Chinese drug maker Sinovac was 78% effective in the prevention of COVID-19 infections.

“Today is the day of hope, the day of life”, João Doria, Governor of the Brazilian State of São Paulo,said at a press conferenceJanuary 7.

But at a press conference on Tuesday, officials delivered a more sober follow-up: Sinovac’s vaccine was, in fact, only 50.4% effective in preventing COVID-19 infections. Officials said the lowered figure represented “very mild” cases of COVID-19 among participants in the phase III vaccine trial in the country that had been omitted in the previous analysis.

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

Ricardo Palácios, medical director of the Butantan Institute, argued that the lower efficacy rate proved that the Brazilian trials of Sinovac were focused on higher risk groups than the Pfizer and Moderna trials and which the institute was using stricter standards. “We added all the possible difficulties [to our trials],” he said.

However, neither Sinovac nor the Butantan Institute have disclosed to the public all the details of its test data, and Sinovac has not responded. Fortunerequesting comments on the new vaccine efficacy figure.

Experts say Fortune that Sinovac’s reduced efficacy rate may not be as big a blow as it seems, and that the vaccine may still prove to be a useful tool to help end the global pandemic. At the same time, Tuesday’s announcement marks the latest in a series of inconsistent conclusions about the potency of the Sinovac vaccine, which has now produced four different efficacy results on three continents. The jarring numbers, combined with the dearth of publicly available data, can make it harder for the company to build public confidence in its doses.

Why has the rate gone down?

The efficacy rate of a vaccine is calculated by comparing the number of COVID-19 infections among the trial volunteers who received the vaccine and the cases among the volunteers in the placebo group. In the trial in Brazil, 85 participants injected the Sinovac vaccine developed COVID-19 infections versus 167 who developed infections in the placebo group, creating an effectiveness rate of just over 50%.

At Tuesday’s press conference, Palácios said Sinovac’s relatively low 50% effectiveness rate was due to its more stringent standard for what counts as 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. (Brazilian authorities have stressed that the public should focus on the vaccine’s 100% effectiveness in protecting against severe cases of COVID-19.) Western vaccine makers generally only included mild, moderate, and severe categories. , and companies like Pfizer are doing further studies to see if the vaccine will prevent asymptomatic cases.

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

There is evidence to support Palácios’ claim that Brazil’s testing standards are more likely to detect mild cases of COVID-19 than tests conducted elsewhere, resulting in a lower efficacy rate.

In the United States Food and Drug Administration’s assessment of Pfizer’s vaccine trials, the agency found that there were as many as 1,594 suspected but unconfirmed cases of COVID-19. As the infections were not confirmed, they were not taken into account in the effectiveness rate of the Pfizer vaccine.

“It is true that other vaccine trials have not been able to confirm so many mild infections [as Sinovac]», Explains Ben Cowling, epidemiologist at the University of Hong Kong. “The [vaccine efficacy] against mild illness could be less than 90% -95% for Moderna and Pfizer. “

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


Cowling warned that Sinovac’s efficacy figure is still based on limited public data and that more comprehensive data publication is needed to fully assess the efficacy of Sinovac’s vaccine.

“I think we really need to consult more comprehensive documentation before we do any evaluation of vaccine performance,” says Cowling. “What we’ve seen so far is pretty limited information.”

It’s not just the new efficacy figure in Brazil and the lack of public data that is confusing the efficacy of Sinovac’s vaccine.

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

Still questions about Sinovac’s vaccine have not stopped the company from rolling it out. He distributed doses of the vaccine outside clinics in China for months, and Indonesia used Sinovac’s vaccine to launch a massive vaccination campaign for 181 million people on Wednesday. Alongside ministers of health and Indonesia’s top Muslim cleric, Indonesian President Joko Widodo received the first dose of Sinovac vaccine on Wednesday in a televised ceremony from his presidential palace.

But there is no guarantee that others will also be willing to take the hit.

“[Transparency] is a concern, ”Huang said. “Data transparency, even for intermediate results, could still be very important in convincing people that the results were really reliable.”

Even with the unknowns lingering, the vaccine can prove useful in immunizing millions of people who have no access to any alternatives.

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

“I was initially surprised at how low that efficiency rate was, but after thinking more about it, the 50% wasn’t that bad,” Huang says. “It may further reduce the risk of COVID infection and be seen as a game changer in the fight against the current pandemic.”

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