Levels of an antibody found in recovered Covid-19 patients fell sharply in 2-3 months after infection for both symptomatic and asymptomatic patients, according to a Chinese study, raising questions about the length of any immunity against the novel coronavirus, Reuters reports.

The research, published in Nature Medicine on June 18, highlights the risks of using Covid-19 ‘immunity passports’ and supports the prolonged use of public health interventions such as social distancing and isolating high-risk groups, researchers said.

Health authorities in some countries such as Germany are debating the ethics and practicalities of allowing people who test positive for antibodies to move more freely than others who don’t.

The research, which studied 37 symptomatic patients and 37 asymptomatic patients, found that of those who tested positive for the presence of the IgG antibody, one of the main types of antibodies induced after infection, over 90% showed sharp declines in 2-3 months.

The median percentage decrease was more than 70% for both symptomatic and asymptomatic patients.

For neutralising serum antibodies, the median percentage of decrease for symptomatic individuals was 11.7%, while for asymptomatic individuals it was 8.3%.

The study was conducted by researchers at Chongqing Medical University, a branch of the Chinese Center for Disease Control and Prevention and other institutes.