Medical research: Asymptomatic patients with COVID-19 may have weaker immune responses

People who have been infected with the coronavirus SARS-CoV-2 but never develop any symptoms of COVID-19 disease may have a weaker immune response to the virus, suggests a paper published in Nature Medicine this week. The study presents a detailed clinical and immunological analysis of 37 asymptomatic patients infected with SARS-CoV-2.

Most people infected with SARS-CoV-2 experience a mild to severe respiratory illness with symptoms that include fever, cough and shortness of breath, which may appear 2–14 days after exposure. However, others who test positive for the infection either are asymptomatic or experience minor symptoms. The clinical details of such patients are not well understood.

Ai-Long Huang and colleagues studied 37 asymptomatic people diagnosed with SARS-CoV-2 infection from the Wanzhou district, Chongqing, China, before 10 April 2020. Of the 37 asymptomatic patients—identified in a group of 178 people with SARS-CoV-2 infection—22 were female and 15 male, with ages ranging from 8 to 75 years (median age, 41 years). The authors 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. Levels of virus-specific 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, levels of neutralizing 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 (cell-signaling proteins). This indicates that the asymptomatic patients may have had a weaker immune response to SARS-CoV-2 infection, suggest the authors.

The authors also observed that IgG levels began to diminish within 2–3 months of infection in a large proportion of the asymptomatic patients, which was determined using a magnetic chemiluminescence enzyme immunoassay test. They argue that this finding, along with previous analyses of neutralizing 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. Additional studies of larger groups of symptomatic and asymptomatic patients are urgently needed to determine the duration of antibody-based immunity.