Descriptive, Retrospective Study of the Clinical Characteristics of Asymptomatic COVID-19 Patients

Abstract

Since the outbreak of coronavirus disease 2019 (COVID-19) in Wuhan, China, it has rapidly spread around the world. Persons with asymptomatic disease exhibit viral shedding, resulting in transmission, which presents disease control challenges. However, the clinical characteristics of these asymptomatic individuals remain elusive. We collected samples of 25 asymptomatic and 27 symptomatic COVID-19 patients. Viral titers of throat swabs were determined by quantitative reverse transcription-PCR (qRT-PCR). COVID-19 IgG and IgM were examined. Complete blood counts were determined, and serum biochemistry panels were performed. Cytokines, including gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), interleukin 2 (IL-2), IL-4, IL-6, and IL-10 were evaluated. T cell, B cell, and NK cell counts were measured using flow cytometry. Although similar viral loads were detected, asymptomatic patients had significantly faster virus turnover than symptomatic patients. Additionally, asymptomatic patients had higher counts of lymphocytes, T cells, B cells, and NK cells. While liver damage was observed in symptomatic patients, as indicated by elevated liver enzymes and decreased liver-synthesized proteins in the blood, asymptomatic patients showed normal liver measurements. Lactate dehydrogenase, a COVID-19 risk factor, was significantly lower in asymptomatic patients. These results suggest that asymptomatic COVID-19 patients had normal clinical indicators and faster viral clearance than symptomatic patients. Lymphocytes may play a role in their asymptomatic phenotype. Since asymptomatic patients may be a greater risk of virus transmission than symptomatic patients, public health interventions and a broader range of testing may be necessary for the control of COVID-19.IMPORTANCE Asymptomatic transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a potential problem for pandemic control through public health strategies. Our results demonstrate that asymptomatic COVID-19 patients have better outcomes than symptomatic patients. This may have been due to more active cellular immune responses and normal liver function. Since asymptomatic patients have no clinical symptoms which can easily prevent timely diagnosis and treatment, they may cause a greater risk of virus transmission than symptomatic patients, which poses a major challenge to infection control. Evidence suggests that nonpharmaceutical public health interventions, like social distancing and face mask ordinances, play important roles in the control of COVID-19. Looking forward, it may be necessary to proceed cautiously while reopening businesses in areas of epidemicity to prevent potential waves of COVID-19 in the future.

Keywords: COVID-19; antibody; asymptomatic; immune response; liver function.

FIG 1

Dynamic changes of anti-SARS-CoV-2 IgM and IgG antibodies in asymptomatic and symptomatic patients. The box plots display anti-SARS-CoV-2 IgM and IgG concentrations at the 25th, 50th, and 75th percentiles. The cutoff value was defined as 10 AU/ml (dotted line) according to the manufacturer’s instructions by using a SARS-CoV-2 IgM and IgG antibody chemiluminescence detection kit. The x axis represents the patient's hospital day. d0 represents the day of hospital admission.

FIG 2

Comparison of lymphocyte counts of symptomatic and asymptomatic COVID-19 patients before and after treatments. Lymphocyte, basophil, and eosinophil counts from symptomatic and asymptomatic patients before and after COVID-19 treatments have been determined. LYM, lymphocytes; BASO, basophils; EOS, eosinophils. Wilcoxon's signed-rank test was used. *, P < 0.05; **, P < 0.01; ***, P < 0.001.

FIG 3

Changes in the cellular immune responses of symptomatic COVID-19 patients before and after treatments. CD3 total T cell, CD4 T cell, CD8 T cell, CD19 B cell, and CD16⁺ CD56⁺ NK cell counts from symptomatic patients before and after COVID-19 treatments were analyzed. Wilcoxon's signed-rank test was used. *, P < 0.05; **, P < 0.01; ***, P < 0.001.