A prominent Chinese respiratory disease expert said it will take two to three years to achieve herd immunity worldwide through vaccination against the coronavirus, and he warned that no country is safe until all countries are safe.

Zhong Nanshan, director of the National Clinical Research Center for Respiratory Disease and an academician of the Chinese Academy of Engineering, made the remarks in his keynote speech at the 20th Conference of the Science Council of Asia in Guangzhou, Guangdong province, on Thursday.

Zhong said one way to achieve herd immunity is through natural infection — infection without interference — which would result in a 70 to 80 percent infection rate and 5 percent mortality. Therefore, he said, "natural immunity is unrealistic, less scientific and inhumane."

Another way to achieve herd immunity is mass vaccination, which requires two to three years of global collaboration, Zhong said.

Currently, all approved vaccines are effective at protecting people from COVID-19, including prevention of hospitalization with serious cases or death.

Typical side effects of vaccinations include pain at the injection site, fever, fatigue, headache, muscle soreness, chills and diarrhea, Zhong said, urging all to be vaccinated.