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Authors: Xiao et al.

Link to paper:

Journal/ Pre-Print: MedRxiv

Tags: Immunology/Immunity, Clinical/ Diagnostics 

Research Highlights 

1. Asymptomatic COVID19 patients have an earlier viral clearance compared to pre-symptomatic and symptomatic patients.

2. Asymptomatic COVID19 patients have a significantly lower IgM level and a mildly lower IgG level compared to pre-symptomatic and symptomatic patients.


The retrospective study based on the medical records of a total of 75 age-matched patients (23 asymptomatic, 33 pre-symptomatic, and 19 symptomatic) from a single medical facility shows that asymptomatic COVID19 patients have a shorter period between the first day of positive nucleic acid test to the first continuous negative test, which the authors claim to represent a faster viral clearance. By comparing the serum antibody levels during the time of hospitalisation and after discharge, the study also shows that asymptomatic patients have a significantly lower IgM level but relatively similar IgG and IgA levels. The authors also found lung damage from 13 of the 23 asymptomatic patients from chest CT scans.

Impact for SARS-CoV2/COVID19 research efforts

Understand the immune response to SARS-CoV2/COVID19

Clinical symptoms and pathogenesis of SARS-Cov2/COVID19

Treatment of SARS-CoV2/COVID19 positive individuals

Study Type

· Patient Case study

Strengths and limitations of the paper

Novelty: First study to attempt to assess the virological and immunological features in asymptomatic COVID19 patients

Standing in the field: Showed again that being asymptomatic when infected by SARS-CoV-2 seems to be related to young age.

Agreed with previous view that asymptomatic COVID19 patients generate lower adaptive immune response during infection.

Abnormal chest CT scans also found for asymptomatic patients in previous studies.

Appropriate statistics: Some statistics are used but many comparisons did not involve statistics but have been done simply by numerical comparison. It is not stated whether repeat experiments were done to assess biological variation, and no mention is made of technical replicates.

Viral model used: SARS-CoV-2 (ShenZhen)

Translatability: May provide limited insights into treatment of asymptomatic COVID19 patients who have a positive nucleic acid test. Suggested chest CT scans as a potential reassuring diagnostic method.

Main limitations:

· No control group with non-infected individuals.

· Each group contains only 19, 23 and 33 patients. A larger sample would have increased study power.

· The definition of RNA negative conversion and communicable period is confusing.

· Neither technical nor biological variation was reported in the vRNA or antibody assays.

· Different age groups might need to be stratified when comparing immune responses, since asymptomatic adults may have very different immunological responses compared to asymptomatic childen/infants.