Most Covid-19 cases in Wuhan had no symptoms, less than half produced antibodies against new infection: Study

Over eight in 10 people in Wuhan, who tested positive for Covid-19 antibodies, were asymptomatic for the disease.
Over eight in 10 people in Wuhan, who tested positive for Covid-19 antibodies, were asymptomatic for the disease.PHOTO: REUTERS

BEIJING - Over eight in 10 people in the Chinese city of Wuhan who tested positive for Covid-19 antibodies were asymptomatic for the disease and less than half developed neutralising antibodies that protect against a future infection, a new Chinese study shows.

The study, which researchers said was the first long-term Covid-19 antibody prevalence study from the epicentre of the outbreak in China, also found that 6.9 per cent had Covid-19 antibodies, indicating a prior infection.

The figure indicates that close to 765,000 people could have been infected in Wuhan, a city of 11.08 million, confirming reports that infection numbers there were far higher than the official tally of around 50,000 cases.

A previous study by the Chinese Centre for Disease Control and Prevention in December found an antibody prevalence rate of 4.43 per cent in Wuhan.

The latest study, published in medical journal The Lancet last Saturday (March 20), looked at seroprevalence, or prevalance of antibodies, of 9,542 people in Wuhan from April to December last year.

The study was done by a group of Chinese researchers from a range of medical and research institutions, including the Chinese Academy of Medical Sciences and Peking Union Medical College.

Researchers had found that a vast majority, or 82 per cent, of those who tested positive for antibodies had not experienced symptoms, pointing out that this was "much higher than the average proportions of 40 to 45 per cent that have been reported worldwide".

The study's lead author, Dr Wang Chen from the Chinese Academy of Medical Sciences, said that the discrepancy was unlikely to be due to recall bias on the part of the study participants, given the efforts by the authorities to identify cases and encourage residents to record their symptoms.

Researchers had also found that only about 40 per cent of those with antibodies had developed neutralising antibodies - the type that protects against a future infection - and the levels of such antibodies were higher in individuals that showed symptoms.

Levels of these neutralising antibodies were stable for at least nine months, they found.

Dr Wang said assessing the proportion of the population immune to Covid-19 was "of utmost importance for determining effective prevention and control strategies".

Researchers said the low-level of immunity, even in the pandemic's epicentre of Wuhan, underscore the importance of vaccines, and that "mass vaccination is needed to effect herd protection to prevent the resurgence of the epidemic".

Dr Jin Dong-Yan, a virology professor at the University of Hong Kong, said the study was important and adds to the growing body of knowledge that actual infection numbers were much higher in Wuhan than official figures.

But he also pointed out that while the study found only 40 per cent of those infected had developed neutralising antibodies, it does not mean that those without had a lack of immunity to the virus.

"It is well possible that they have memory B and T cells against Sars-CoV-2," he said, referring to the virus that causes Covid-19.

Memory B and T cells are long-lived cells that are developed after a prior infection, and lie dormant until infection by the pathogen again, whereupon they trigger a swift immune response.

"When these people are infected or vaccinated, if they develop neutralising antibodies quickly, they might also be fully protected against Sars-CoV-2," said Prof Jin.