People wearing face masks walk on a main shopping street in Wuhan, Hubei province, March 30, 2020. (WANG JING / CHINA DAILY)

About 40 percent of the people in Wuhan, Hubei province, who caught COVID-19 had antibodies that could offer protection against reinfection for at least nine months, according to an article published in the medical journal The Lancet last week.

The study said the adjusted COVID-19 positive rate in the Chinese city hit hardest by the virus last year was just 6.9 percent, indicating that only a small portion of the city's population was infected during the epidemic.

The antibody levels in asymptomatic patients were found to be lower than those in confirmed patients in the study. The results may help facilitate precise COVID-19 infection prevention in the future, said co-author Ren Lili, from the academy's Institute of Pathogen Biology and Peking Union Medical College

"Assessing the proportion of the population that has been infected with COVID-19 and who are immune is crucial for determining effective prevention and control strategies to reduce the likelihood of a future resurgence of the pandemic," said Wang Chen, lead author of the article and president of the Chinese Academy of Medical Sciences and Peking Union Medical College.

The study－the first long-term seroprevalence survey in Wuhan－tested more than 9,500 residents in the city's 13 districts for COVID-19 antibodies after its lockdown was lifted in April. Follow-up blood-sample tests were conducted in June and between October and December to examine if antibodies were present.

Previous studies in many countries showed that the infected COVID-19 population, as estimated by the positive rate of serum antibodies, is much higher than the actual infected cases. The new study suggests this may be primarily because most of the infected people were asymptomatic or did not get tested or treated due to their mild infections.

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Further, the antibody levels in asymptomatic patients were found to be lower than those in confirmed patients in the study. The results may help facilitate precise COVID-19 infection prevention in the future, said co-author Ren Lili, from the academy's Institute of Pathogen Biology and Peking Union Medical College.

"Little is known of the durability of immune responses against COVID-19 over a long period," Ren said. "In our study, we found that the proportion of participants with antibodies against the virus was sustained for at least nine months. Importantly, we found that neutralizing antibody titer remained stable for at least nine months."

Some of the researchers who worked on the study were from the Wuhan Disease Control and Prevention Center and the Chinese Center for Disease Control and Prevention.

They said that understanding seroprevalence and how antibody levels change over time in Wuhan would help inform their vaccination strategies, with their findings indicating that mass vaccination is needed to protect against future resurgences of the virus.

The study "underscores the remarkable achievement of the Chinese public health system in controlling the Wuhan outbreak of COVID-19 at a time when testing, tracing, and treatment resources were much less developed", Richard Strugnell, a renowned expert in microbiology and immunology from Australia's Doherty Institute, wrote in an accompanying article in the journal commenting on the Chinese team's findings.

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"It is an important milestone in the description of SARS-CoV-2 infection and our understanding of immunity in the pandemic," he wrote.

Xinhua