Health authorities in Brazil have released detailed results of a large-scale human trial for the CoronaVac vaccine, the first time complete trial data for a Chinese-developed COVID-19 shot has been shared with the public.

In a non-peer-reviewed preprint paper published Monday, scientists from São Paulo’s Butantan Institute, two representatives from vaccine developer Sinovac, and researchers and doctors from more than a dozen participating institutions gave a demographic breakdown of the trial’s 12,000-plus participants, efficacy rates for preventing infections of varying degrees of severity, detailed explanations of adverse reactions, and a hypothesis for how CoronaVac’s efficacy might be improved.

As was previously announced at a January press conference, CorvonaVac was found to be 50.7% effective in preventing symptomatic COVID-19, 83.7% effective against “cases requiring assistance,” and 100% effective in preventing moderate or severe cases. Six trial participants who contracted severe COVID-19 were all in the placebo group.

CoronaVac has the lowest reported efficacy rate of any coronavirus vaccine, though it still meets the World Health Organization’s approval threshold of 50%. By contrast, Beijing-based Sinopharm says its vaccine is around 79% effective, while the Moderna and Pfizer-BioNTech vaccines both have efficacy rates above 94%.

Over the weekend, Gao Fu, director of the Chinese Center for Disease Control and Prevention, acknowledged the relatively low efficacy rates of the country’s COVID-19 vaccines and said China would consider mixing them to boost efficacy rates.

However, the Brazilian scientists appeared satisfied with CoronaVac’s trial performance. “Efficacy to prevent any symptomatic COVID-19 started at 50.7% and became more extensive as disease severity increased,” they wrote, adding that the Chinese vaccine was demonstrated to be “safe” and “well-tolerated.”

A total of 12,396 people, all of them health care professionals, participated in the Brazil trial. They received two injections — either placebo or vaccine — at least two weeks apart. After the trial was expanded to include people aged 60 or above, these elderly accounted for 5.1% of the group, which had an average age of 39.5.

There have been worries in China about whether the country’s vaccines are safe and effective for people aged 60 or above, or for women planning to have children. A previous small-scale trial among seniors had shown the shot to be safe for this population. But because of a lack of data on pregnant or lactating women, questions of safety and efficacy among these groups remained.

The CoronaVac trial did not provide many answers. It included only 632 elderly participants, and pregnant or lactating women were ineligible due to ethical concerns. However, the Chinese government has since said that these groups can safely get vaccinated.

The trial’s vaccine group reported more adverse reactions than the placebo group, mostly pain around the injection site. Headache and fatigue were reported similarly by both groups. There were no severe reactions related to the vaccine. “The observed safety and tolerability profiles were outstanding,” the authors wrote.

Three participants died during the trial. Two people in the placebo group died of a heart attack and COVID-19, respectively, while one person in the vaccine group died from a medication overdose — a “serious adverse event” that was not related to the vaccine, the authors said.

The trial was designed with an interval of two weeks between shots, but the authors found that delaying the second injection could potentially improve the vaccine’s efficacy.

“Although there were a limited number of participants in this study having doses with an interval of 21 days or higher, there was a trend to higher efficacy,” they wrote, referring to the more than 700 vaccine group participants who followed this regimen. “Taken together, these data suggests (sic) that it is advisable to encourage longer intervals between doses, i.e., 28 days.”

Since the trial’s completion, the Butantan Institute has delivered 38.2 million doses of CoronaVac to Brazilian health authorities. As of April 7, Sinovac had distributed a further 180 million doses to around 30 mostly low- and middle-income countries.

In February, China’s drug regulator granted conditional approval to four domestically developed coronavirus vaccines including CoronaVac. The other three have not released data from their clinical trials.

Editors: Kevin Schoenmakers and Ye Ruolin.

(Header image: A medical worker receives a dose of Sinovac’s COVID-19 vaccine at a hospital in São Paulo, Jan. 17, 2021. Patricia Monteiro/Bloomberg via Getty Images/People Visual)