About 40 percent of COVID-19 positive patients in Wuhan, capital of central China's Hubei Province, have antibodies that can offer protection against being reinfected with the virus for at least nine months, said a new article published in the medical journal The Lancet this week.

According to the study, the adjusted COVID-19 positive rate in the city hardest hit by the virus earlier last year was just 6.9 percent, indicating that only a small proportion of the city population was infected after the epidemic outbreak.

"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 first long-term seroprevalence survey in Wuhan, the study tested COVID-19 antibodies in more than 9,500 residents living across all 13 districts after the city's lockdown was lifted in early April of 2020. 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 due to the fact that most of the infected people were asymptomatic or did not get tested or treated due to their mild infections.

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

"Little is known of the durability of immune responses against COVID-19 over a long period. 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 titers remained stable for at least nine months," Ren said.

Among the researchers working on the study are some 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 latest findings.

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