People who have had coronavirus without any symptoms may have a weaker immune response to Covid-19, research suggests.
Most people infected with the virus experience a mild to severe respiratory illness with symptoms that include a fever and cough.
It may take up to 14 days for these symptoms to appear.
However, others who test positive for the infection are either asymptomatic – never develop any symptoms – or experience minor symptoms.
But little is known about these patients.
The World Health Organisation (Who) estimates that up to 41% of people infected with Covid-19 may be asymptomatic.
Researchers in China studied 37 asymptomatic people diagnosed with Sars-CoV-2 infection from the Wanzhou district of Chongqing before April 10 2020.
Identified in a group of 178 people with the virus, 22 of them were female and 15 male, with ages ranging from eight to 75.
The authors of the study, published in Nature Medicine, found that these patients had a median duration of viral shedding of 19 days, compared with 14 days in a group of 37 symptomatic patients.
Viral shedding refers to how much virus someone sheds, or excretes.
Researchers found that levels of virus-specific antibodies were significantly lower in the asymptomatic group than in the symptomatic group during the acute phase of infection, when the virus could be detected in the respiratory tract.
Antibodies are produced by the immune system when it is being attacked, and indicate whether someone has had the virus.
Eight weeks after the patients were discharged from the hospital, levels of the antibodies decreased in 81.1% of asymptomatic patients, compared with 62.2% of symptomatic patients.
In addition, asymptomatic patients had lower levels of 18 pro- and anti-inflammatory cytokines – proteins that are secreted by specific cells of the immune system.
This indicates that the asymptomatic patients may have had a weaker immune response to Sars-CoV-2 infection, the authors suggest.
Ai-Long Huang, from Chongqing Medical University, and colleagues also observed that antibody levels began to diminish within two to three months of infection in a large proportion of the asymptomatic patients.
They argue that this finding, along with previous analyses of neutralising antibodies in patients recovering from Covid-19, highlights the potential risks of using immunity passports and supports the continuation of public health interventions and widespread testing.
They add that additional studies of larger groups of symptomatic and asymptomatic patients are urgently needed to determine the duration of antibody-based immunity.
The authors write: “These data suggest that asymptomatic individuals had a weaker immune response to Sars-CoV-2 infection.
“The reduction in IgG and neutralising antibody levels in the early convalescent phase might have implications for immunity strategy and serological surveys.”
They add: “Of the 178 laboratory-confirmed patients, 37 who never developed any symptoms throughout the disease course were included in this study.
“Our data showed that 20.8% of these patients had asymptomatic infections.
“However, this might not be an accurate estimation of the proportion of asymptomatic infections in the general population owing to the fact that asymptomatic infections were identified from those who were at high risk for infection and not from a random sample of people.
“Therefore, the proportion of asymptomatic infections needs to be determined through population screening.”