COVID-19 patients who do not show any symptoms of the disease may have a weaker immune response to the virus, according to a new study which highlights the risks of using ‘immunity passports’ to certify that a person has recovered from COVID-19, and is fit for travel and work.
The research, published in the journal Nature Medicine, presents an analysis of the clinical and immunological manifestations of 37 asymptomatic patients infected with the novel coronavirus, SARS-CoV-2.
It found that these patients had an average duration of viral shedding of 19 days, compared with 14 days in another group of 37 patients who showed symptoms of the disease.
According to the scientists from the Chongqing Medical University in China, most people infected with SARS-CoV-2 experience a mild to severe respiratory illness with symptoms that include fever, cough, and shortness of breath that may appear 2-14 days after exposure.
However, they said several who test positive for the infection may either experience minor symptoms or be asymptomatic.
In the study, the researchers studied 37 asymptomatic people diagnosed with SARS-CoV-2 infection from the Wanzhou district in China, before 10 April 2020.
Of these asymptomatic patients from a group of 178 people with SARS-CoV-2 infection, 22 were female and 15 male, with ages ranging from 8 to 75 years, the scientists said.
“In comparison to symptomatic patients, the asymptomatic group had a significantly longer duration of viral shedding, with a viral shedding time of 19 days,” they wrote in the study.
According to the study, the levels of virus-specific immune system molecules called IgG 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.
Eight weeks after the patients were discharged from the hospital, the researchers said the levels of neutralising antibodies decreased in more than 8o per cent of asymptomatic patients, compared with about 62 per cent of symptomatic patients.
They added that the asymptomatic patients also had lower levels of 18 pro- and anti-inflammatory cell-cell signalling proteins called cytokines.
Based on these observations, the scientists believe that the asymptomatic patients may have had a weaker immune response to SARS-CoV-2 infection.
The IgG antibody levels in the patients also began to diminish within three months of infection in a large proportion of the asymptomatic patients, the study noted.
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, the researchers said.
They believe that further research in larger groups of symptomatic and asymptomatic patients is urgently needed to determine the duration of antibody-based immunity.
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