Recovered coronavirus patients can have very low levels of antibodies in their system, researchers have found, in a discovery that could hamper the development of immunity tests.
A team from Fudan University in China analysed blood from 175 patients discharged from the Shanghai Public Health Clinical Centre and found nearly a third had surprisingly few antibodies.
In 10 patients, antibodies could not be detected at all, which could also place them at greater risk of a secondary infection, the researchers warned.
"Whether these patients were at high risk of rebound or reinfection should be explored in further studies," the authors wrote in an article on the preprint website Medrxiv.org, an online platform for early research which has not yet been peer-reviewed.
All of the patients had recently recovered from mild symptoms of the disease and most of those who had low antibody levels were young.
Levels rose with age, with people in the 60-85 age group displaying more than three times the number of antibodies as people in the 15-39 age group, in line with suspicions that the virus is most deadly when it triggers an extreme immune response.
British experts said the results showed why the Government was struggling to produce a reliable antibody test, as it may need to be far more sensitive than previously thought.
Paul Hunter, professor in medicine at the University of East Anglia, said: "This has relevance to the current debate about antibody tests in the UK. If many people only produce low levels of antibodies to Sars-CoV-2 [the scientific name of the virus behind Covid-19] then any community test would need to have high sensitivity. This provides further insight about why community antibody tests in the UK have not yet been authorised for use."
However, Eleanor Riley, professor of immunology and infectious disease at the University of Edinburgh, said the findings were reassuring and in line with other respiratory diseases.
"A small number of people did not produce detectable neutralising antibodies but they all recovered, suggesting that there are other types of antibodies or other components of the immune response that can also fight the infection," she said.
"Levels of all the antibodies analysed were higher in older individuals than in younger people and also correlated with markers of disease severity.
"Although viral load data were not available (a major limitation of the study) this would be consistent with antibody levels being higher in those recovering from more severe disease, which may be due to higher viral loads."