Medical authorities in China have said a drug used in Japan to treat new strains of influenza appeared to be effective in coronavirus patients, Japanese media said on Wednesday.

Zhang Xinmin, an official at China’s science and technology ministry, said favipiravir, developed by a subsidiary of Fujifilm, had produced encouraging outcomes in clinical trials in Wuhan and Shenzhen involving 340 patients.

“It has a high degree of safety and is clearly effective in treatment,” Zhang told reporters on Tuesday.

Patients who were given the medicine in Shenzhen turned negative for the virus after a median of four days after becoming positive, compared with a median of 11 days for those who were not treated with the drug, public broadcaster NHK said.

In addition, X-rays confirmed improvements in lung condition in about 91% of the patients who were treated with favipiravir, compared to 62% or those without the drug.

Fujifilm Toyama Chemical, which developed the drug – also known as Avigan – in 2014, has declined to comment on the claims.

Shares in the firm surged on Wednesday following Zhang’s comments, closing the morning up 14.7% at 5,207 yen, having briefly hit their daily limit high of 5,238 yen.

Doctors in Japan are using the same drug in clinical studies on coronavirus patients with mild to moderate symptoms, hoping it will prevent the virus from multiplying in patients.

But a Japanese health ministry source suggested the drug was not as effective in people with more severe symptoms. “We’ve given Avigan to 70 to 80 people, but it doesn’t seem to work that well when the virus has already multiplied,” the source told the Mainichi Shimbun.

The same limitations had been identified in studies involving coronavirus patients using a combination of the HIV antiretrovirals lopinavir and ritonavir, the source added.

In 2016, the Japanese government supplied favipiravir as an emergency aid to counter the Ebola virus outbreak in Guinea.

Favipiravir would need government approval for full-scale use on Covid-19 patients, since it was originally intended to treat flu.