The COVID-19 pandemic cannot be controlled by the herd immunity strategy and vaccines will be the key to ultimately tackling the virus, said Zhong Nanshan, a prominent Chinese expert in respiratory diseases.

There is no evidence that a person becomes immune forever to coronavirus-related illnesses after having been infected once, he said at a news conference in Guangzhou on Wednesday.

China has been moving quickly in vaccine development and international cooperation is necessary in this regard, said Zhong, who is also one of the leading specialists tackling the outbreak in China.

No matter which country produces the vaccine first, it definitely cannot supply enough to the entire world, he said, adding that it will need to come from multiple sources.

Responding to a question on the origin of the novel coronavirus, Zhong said that although Wuhan first reported the outbreak there is no evidence that the city is the source of the coronavirus that causes COVID-19.

A lot more needs to be understood about COVID-19 and international cooperation is important in fields such as drug development, he said.

His team plans to issue a report next week on clinical medication in treating COVID-19 patients.

Zhong stressed the importance of nucleic acid tests for people returning from foreign countries, as imported cases now account for the majority of the new cases in China.

The virus remains highly contagious and some infected people have shown no symptoms when entering China.

Meanwhile, Chinese people overseas are being urged not to return to Guangdong province during Qingming Festival, or Tomb Sweeping Day holiday, as provincial health authorities further tighten the response to the novel coronavirus, the provincial health commission said at a news conference on Wednesday.

Qingming Festival, which begins on April 4, is usually the peak time for millions of overseas Chinese to return to Guangdong, said Ji Leqin, an official with the provincial health commission.

"At a time of rising numbers of confirmed imported COVID-19 cases, people are being gently urged not to return home," Ji said.

As of Wednesday afternoon, Guangdong had confirmed 20 imported cases of coronavirus infection, the health commission said.

Ji said that people returning from countries hit hard by the coronavirus will be required to undergo medical observation for 14 days.

"Paying respect to ancestors on online platforms is encouraged," Ji said.

According to Fang Qiaping, deputy director of the Guangdong Provincial Civil Affairs Department, the province will not organize any group activities to honor ancestors during this year's festival.

"We will launch online activities instead," Fang said, adding that cemeteries will be closed during the festival.