About a quarter of China’s new confirmed cases and almost all of those outside the epidemic’s epicentre in Wuhan originated outside the country on Friday, according to official data.

Most of these cases, which include infections of Chinese nationals who caught the virus abroad, were in China’s northwestern Gansu province, among quarantined passengers who entered the provincial capital of Lanzhou on commercial flights from Iran between 2 March and 5 March.

In addition to the growing risks of imported infections, China’s anti-virus efforts also face the challenge of trying to get migrant workers back to work by early April.

So far, 78 million migrant workers, or 60% of the total who left for the Lunar New Year holiday, have returned.

“I must stress, that we’re still at a critical juncture in terms of epidemic prevention and control,” said Yang Wenzhuang, an official of China’s National Health Commission (NHC).

“The risk of contagion from increased population flows and gathering is increasing … We must not relax or lower the bar for virus control.”

Mainland China had 99 new confirmed cases of coronavirus infections on Friday, NHC said on Saturday, down from 143 cases a day earlier and marking the lowest number since Jan. 20, when the NHC started to publish nationwide figures.

Outside of central China’s Hubei province, there were 25 new confirmed cases reported on March 6, of which 24 came from outside China.

The capital Beijing reported four new cases on Friday, of which three came from Italy, according to a notice from the Beijing health commission posted on its official Weibo account on Saturday.

The fourth was a case of a recovered patient testing positive again. There were also three cases in Shanghai that originated abroad, and one in Guangdong province on Friday, according to the National Health Commission.

The total nationwide number of cases that originated outside China reached 60 as of the end of Friday.

For the second day in a row, there were no new infections in Hubei outside of the provincial capital of Wuhan, where new cases fell to the lowest level since 25 Jan.

Special institutions like prisons, detention centres and nursing homes in Wuhan, which have seen nearly 1,800 confirmed cases as of March 5, still have potential risks in virus control and prevention, the Communist Party’s Politics and Law Commission said on Saturday.

The total number of confirmed cases in mainland China so far is 80,651.

The death toll from the outbreak in mainland China was 3,070 as of the end of Friday, up by 28 from the previous day. The central province of Hubei, the epicentre of the outbreak, reported 28 new deaths. In the provincial capital of Wuhan, 21 people died.