The world might need two to three years to reach herd immunity against COVID-19 through vaccination, China's top respiratory expert Zhong Nanshan said on Thursday at the Science Council of Asia Conference in south China's Guangzhou City.

He added that it will only be attainable with global cooperation.

The other way to meet the goal of herd community, Zhong said, is natural immunity. It can only be achieved when 70 to 80 percent of population have been infected with a death rate of five percent, which is "neither realistic nor scientific nor humane."

Zhong stressed the vaccines work effectively on preventing death and severe illness caused by COVID-19. The side effects of the vaccine include soreness, fever, fatigue, muscle ache, shivering and diarrhea.

According to Zhong, when the efficacy of vaccines is about 70 percent, the vaccination rate needs to exceed 80 percent to reach global herd immunity. If the efficacy can reach up to 80 percent, global herd immunity can succeed when the vaccination rate goes over 70 percent.

"No country is safe until all countries are safe," Zhong added.

Days ago, China's Sinopharm COVID-19 vaccine was approved by World Health Organization for "emergency use," validating the vaccine's efficacy and effort for wider use. The other Chinese vaccine SinoVac still awaits approval for global use.

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