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Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections

Our take —

Asymptomatic individuals may shed viral RNA for significantly longer than symptomatic individuals, but the infectivity of this viral shedding is unknown. SARS-CoV-2 IgG antibody response is lower among asymptomatic infections than what is observed among symptomatic individuals; neutralizing antibody levels appear to decrease around 2-3 months post-infection in symptomatic and asymptomatic individuals, but it is unknown what effect this may have on reinfection with SARS-CoV-2. Lastly, timely and thorough serological testing is important in order to determine the true infection rate in the general population.

Study design

Case Series; Other

Study population and setting

This study included 37 asymptomatic individuals from the Wanzhou District of China with positive RT-PCR results for SARS-CoV-2, as well as 37 mildly symptomatic age, sex and comorbidity matched controls. The median age of the asymptomatic individuals was 41 years, and 22/37 were female.

Summary of Main Findings

In terms of radiologic and laboratory findings, the asymptomatic patients had no pleural effusions, air bronchogram signs, or enlarged lymph nodes, which are usually typical symptoms of COVID-19. 11/37 asymptomatic individuals exhibited focal ground-glass opacities in their chest CT scans, 10/37 had stripe shadows and/or diffuse consolidation, while 16/37 showed no abnormalities. The median duration of viral shedding in the asymptomatic group was 19 days, which was significantly longer than the 14 day median of the symptomatic group. In addition, a majority of asymptomatic individuals were positive for SARS-CoV-2 specific IgG and IgM antibodies, however, levels were lower than in the symptomatic group, with IgG levels being significantly higher in symptomatic patients. Neutralizing antibodies began to decrease within 2-3 months post-infection. Asymptomatic individuals additionally had a reduced inflammatory response compared to symptomatic individuals, characterized by lower levels of circulating pro-inflammatory cytokines and chemokines.

Study Strengths

Asymptomatic patients used in the study were carefully selected from a group of over 2,000 individuals under quarantine in Wanzhou District. Sex-, age-frequency and comorbidity matched mild symptomatic patients were used for comparison to asymptomatic patients, as well as sex- and age-frequency matched control individuals with negative RT-PCR results for SARS-CoV-2. Additionally, a wide range of metrics were studied, including laboratory and radiographic findings, viral shedding, antibody levels, as well as cytokines and chemokines.


The measurable viral shedding observed here via RT-PCR does not necessarily indicate viral infectivity – further evaluation is needed to determine the amount of culturable virus in the detectable viral load. The population of asymptomatic patients detected in the Wanzhou District case finding may not reflect the overall population. Some cases may have been missed by RT-PCR testing, and these individuals may require serological testing, as well. Additionally, the sample size is limited and the population here was not selected from a random population sample, but rather from a pool of people considered high-risk.

Value added

This study is one of the first of its kind, investigating the clinical features and immunological responses found in asymptomatic COVID-19 patients.

This review was posted on: 18 July 2020