A sero-epidemiological survey of the Chinese mainland showed the COVID-19 infection rate was at a low level, said the Chinese Center for Disease Control and Prevention (CDC) on Monday. 

People who have come into contact with coronavirus-infected patients have a higher number of antibodies than people who haven't, and the antibody rate of middle-aged and older people is higher than other age groups, according to the survey.

The survey, which enrolled over 34,000 people living in communities across China, was used to measure the proportion of people infected by the novel coronavirus. Blood samples were tested to identify people who have antibodies against SARS-CoV-2, the virus that causes the COVID-19 disease. Those who tested positive may suggest prior infections.

The survey found the antibody rate among samples collected in Wuhan City, the epicenter of China's coronavirus outbreak in Hubei Province, is 4.43 percent, while the rate in other neighboring cities within the province is 0.44 percent.

However, among the more than 12,000 samples collected outside Hubei Province, only two people were found to have coronavirus antibodies. This demonstrates that COVID-19 exposure in the community is uncommon, indicating no widespread community transmission.

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Some European countries have conducted similar sero-epidemiological surveys. In Spain, the result of 8,902 samples collected between April 17 and June 2 showed the overall prevalence of antibody sero-positivity in the general population was 17.1 percent.

Scientists in Switzerland have also enrolled 2,766 participants from 1,339 households between April 6 and May 9. They found the sero-positivity in their country had risen from 4.8 percent to 10.8 percent during that period.

Both studies have been published in the renowned medical journal The Lancet.

According to the Chinese CDC, the sero-epidemiological survey can help scientists compare data across different age groups and different locations, so as to evaluate the effectiveness of COVID-19 control measures in the long haul.

"Rigorous research is critical to ensuring a coordinated, effective national-level outbreak response," said Professor Leo Yee Sin, executive director at the National Centre for Infectious Diseases and chair of the COVID-19 Research Workgroup in Singapore.

"In the case of COVID-19, of which much remains unknown, a swift response in clinical aspects as well as research is especially crucial to outpace the virus' rapid spread," he added.