A medical worker administers a dose of COVID-19 vaccine to a child at a vaccination site in Panggezhuang Town of Daxing District in Beijing, capital of China, Nov. 4, 2021.Photo: Xinhua

Chinese top respiratory expert Zhong Nanshan said on Thursday that, in theory, China has built a certain level of herd immunity as more than 83 percent of the country's population has been vaccinated.

He made the remarks at a forum in Guangzhou, South China's Guangdong Province. 

Herd immunity could be reached when a certain amount of the population gets vaccinated, Zhong explained, calling for more Chinese residents to take booster shots.

China kicked off its vaccination program starting with high-risk groups in December, 2020.

Data released by the National Health Commission show that more than 1.2 billion people on the Chinese mainland have been fully vaccinated as of December 28, 2021, accounting for more than 85.6 percent of the country's population.

When the majority of a population is immune to an infectious disease, this provides indirect protection or population immunity, also called herd immunity or herd protection, to those who are not immune to the disease, according to a report by the Bloomberg School of Public Health of Johns Hopkins University.  

Depending how contagious an infection is, usually 50 to 90 percent of a population needs immunity before infection rates start to decline, according to the report.

Regarding the SARS-CoV-2 virus that causes COVID-19, the figure that many epidemiologists offered at the early stage of the pandemic was 60 to 70 percent. But the threshold keeps being pushed further by epidemiologists as the pandemic worsens and the virus mutates into more transmissible variants.

Top US pandemic advisor Anthony Fauci told media in December 2021, that he believes that it may take close to 90 percent immunity to bring the virus to a halt.