China's Sinovac vaccine is 50.7% effective against COVID-19, just reaching the threshold to be a vaccine worth using, a major trial showed

• Sinovac's COVID-19 vaccine CoronaVac was 50.7% effective against symptomatic COVID-19 in a Brazil trial.
• The benchmark for a vaccine worth using is 50%.
• The trial of Sinovac's CoronaVac vaccine tested the shot in more than 12,000 health workers.

The COVID-19 vaccine from Chinese biotech Sinovac is about 50% effective at preventing symptomatic coronavirus, a late-stage trial in Brazil showed.

The two-dose CoronaVac vaccine was safe and 50.7% effective against symptomatic COVID-19, Sinovac's Brazilian partner, Butantan, said in a preprint posted Sunday, confirming top-line results announced by the group in a press release in January.

The World Health Organization (WHO) said April 2020 that a vaccine that was 50% effective in trials was worth using.

Butantan said that "efficacy to prevent any symptomatic COVID-19 started at 50.7% and became more extensive as disease severity increased." The study authors found CoronaVac was 100% effective against severe COVID-19, but the number of cases was too small for them to be confident in the figure.

The authors noted higher efficacy with more time between doses - 21 days rather than 14 - but didn't provide exact figures.

The study tested the vaccine in more than 12,000 health workers across 16 sites in Brazil.

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The vaccine was generally well tolerated. Some people had pain at the injection site and muscle aches.

The study hasn't been scrutinized by experts in a peer review.

Despite the positive results, George Fu Gao, head of the Chinese Center for Disease Prevention and Control, said Saturday that the vaccine wasn't as good as expected.

In Turkey, CoronaVac was 83.5% effective in a study of more than 10,000 participants aged between 19 and 59.

The Brazil study included people over 60 - about 5% of participants - and took place in a country where coronavirus cases are surging.

Brazil has more than 330 new cases per million every day, compared with 206 in the US and 22 in the UK, according to Oxford University's Our World in Data.

Butantan's study doesn't tell us whether or not CoronaVac works against P.1, a variant that has spread in the country: Data collection ended on December 16, and P.1 was first detected in Brazil on December 4.

If it turned out to be variants making CoronaVac less effective, booster shots made by mixing the vaccine with others could help, Gao said.

The vaccine costs about $30 per dose, according to Chinese state media, and can be stored at normal fridge temperatures, so it could be a useful option for low- and middle-income countries struggling to secure vaccines. The WHO said Friday that low income countries had received just 0.2% of the 700 million jabs administered worldwide.

More than 180 million CoronaVac doses have already been sent to 30 low-and-middle income countries, including Brazil, Chile, and the Philippines, according to Butantan.