Famed Chinese respiratory disease expert Zhong Nanshan urged sustained international collaboration in COVID-19 response on Friday, and greater attention dealing with asymptomatic cases.

Zhong, an academician at the Chinese Academy of Engineering, made his remarks in a congratulatory letter to the Program of the Congress on COVID-19: Pandemic of the 21st Century that opened in Guangzhou, capital of Guangdong province.

"COVID-19 is still spreading globally, and nations and regions around the world should join hands to fight against the pandemic and formulate long-term response strategies," he said. "China and Russia, both adhering to the concept of a community of shared future for mankind, have good cooperation in epidemic prevention and control, clinical treatment and drug research and development, forming a model of global anti-pandemic cooperation."

He said he hoped this international academic exchange would continue to help build a deep and effective platform for the prevention and control of the pandemic in China and Russia, help seek more scientific and effective pandemic prevention and control measures and contribute more to an early end of the spread of coronavirus around the world.

Zhong also urged relevant departments to more carefully handle asymptomatic sufferers who now account for 20 to 40 percent of total cases.

"Asymptomatic sufferers have strong infectivity," he said via video while addressing the opening ceremony of the international exchange.

Regarding response to infection clusters with undetermined sources, he said universal testing among the communities affected is necessary.

He added that cured patients' reinfection is possible, but not common.

He shared two possible explanations for the phenomenon: the first is the antibody produced after the first infection drops rapidly; the second is there is no or only a very weak antibody response during first infection, which cannot resist a second.

Vladimir Chulanov, chief infection disease specialist of the Ministry of Healthcare of the Russian Federation, said China has done a good job fighting COVID-19.