Chinese vaccine developer Sinovac Biotech Ltd. said that the low efficacy rate of 50.38% from its final-stage trial in Brazil is due to trial participants being medical workers facing a high risk of contracting Covid-19.

In contrast, western vaccines like that developed by Pfizer Inc. and BioNTech SE were tested among volunteers including people from the general population, some of whom were less exposed to the virus in their daily lives compared to medical workers.

The defence from Sinovac Chief Executive Officer Yin Weidong at a press briefing in Beijing on Jan 13 followed growing confusion as trials conducted in Brazil, Indonesia and Turkey released four different efficacy rates for the same vaccine.

See: Singapore to open special Covid-19 vaccination centres

Sinovac’s shot has been ordered by places like Singapore and Hong Kong, and the President of Indonesia just received it on live television in a show of confidence designed to persuade people to take it. Chinese-developed vaccines have faced scepticism over a lack of safety and efficacy transparency.

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The Bejing-based company said last July that its Phase III trial in Brazil would recruit nearly 9,000 health-care professionals working in specialised Covid-19 facilities across several states. The trial was later expanded and eventually recruited more than 13,000 people, who were equally split between vaccinated and placebo groups.

See also: China could well be the winner of the Covid-19 vaccine race

It is unclear if the other participants beyond the initial 9,000 were also medical workers.

Sinovac’s Brazil partner Butantan Institute said last week that the vaccine was 78% effective in preventing mild cases of Covid-19 and 100% effective against severe and moderate infections. But the overall rate, which also includes very mild cases that did not require medical help, is actually 50.38%, it later said.

See also: In coronavirus vaccine race, China inoculates thousands before trials are completed

Indonesia’s vaccine rollout will provide a crucial test of how effective the Sinovac shot is, as the country with Southeast Asia’s worst outbreak aims to inoculate 181.5 million people by March 2022.

Yin said the company has received new orders from Malaysia and the Philippines.