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Viral load dynamics in transmissible symptomatic patients with COVID-19

Our take —

This study, which was posted as a preprint and had not yet been peer-reviewed, leveraged epidemiological and clinical data from 28 patients with laboratory-confirmed SARS-CoV-2 from Toyama University Hospital in Japan. This study suggested that viral loads may be higher for patients with SARS-CoV-2 who develop symptoms than people who never develop symptoms, higher for adults than children, and higher among symptomatic cases who transmit to others compared to symptomatic cases who do not transmit to others. This study included a small number of participants, which means that the findings may represent these specific patients but may not be a universal representation of viral load dynamics generally. Additionally, the data collection period may differ between categories of cases which may introduce bias.

Study design

Case Series

Study population and setting

This study leveraged epidemiological and clinical data from 28 immunocompetent laboratory-confirmed patients with SARS-CoV-2 infections from the Toyama University Hospital in Japan. Data were obtained from nasal swab specimens collected over the course of infection, medical charts, and structured telephone interviews. Of the 28 patients, 14 did not transmit the virus onto others (i.e., non-index patients) and 14 eventually transmitted the virus to someone else (i.e., were index patients that caused secondary cases). Overall, 18 of the patients had symptoms and 10 patients did not report symptoms.

Summary of Main Findings

The median viral load at the initial sample collection was higher among adults than in children (2.3 vs. 0.9 log copies/μL, p=0.02); and was higher in patients with symptoms than without (2.8 vs. 0.9 log copies/μL, p<0.01). Among those with symptoms, viral loads peaked shortly after symptoms began, followed by a gradual decrease. The estimated median time between symptom onset and viral clearance was longer in patients who transmitted SARS-CoV-2 to others (21 days [range: 15–31]) than those who did not transmit onward (10 days [range: 9-26]), but this was not a statistically significant difference (p=0.09). Similarly, among symptomatic cases, median viral load at the time of sample collection was higher among patients who transmitted onward compared to those who did not, but not significantly so (3.1 [95% CI: 1.6 to 5.2] vs. 1.9 [95% CI: –0.4 to 4.6] log copies/μL). However, when applying a non-linear regression one-phase decay model over time to symptomatic participants, the authors estimated that viral loads from patients who transmitted onward were significantly higher than those who did not at the time of symptom onset (6.6 [95% CI: 5.2 to 8.2] vs. 3.1 [95% CI: 1.5 to 4.8] log copies/µL, respectively).

Study Strengths

This study measured multiple viral loads across time (i.e., serial assessment) to show differences in how viral load changes during the course of infection with SARS-CoV-2 (i.e., viral load dynamics) in both patients who eventually transmitted the disease to at least one other person and patients who did not transmit to anyone.


The sample size of this study was small and several comparisons did not have enough participants to show statistical differences. Viral load levels at the initial sample collection may also not have been directly comparable between symptomatic and asymptomatic patients if, for example, asymptomatic cases were identified later in in their infection than symptomatic cases. Furthermore, cases with COVID-19 symptoms in this study received treatment, including combinations of antivirals and antibiotics, which may have changed the observed viral load had they not been on treatment.

Value added

This study provided insight on how the amount of SARS-CoV-2 virus changes during the course of infection (i.e. viral load dynamics) among patients with SARS-CoV-2 who transmitted to at least one other person (index cases) and patients with SARS-CoV-2 who did transmit to anyone (non-index cases).

This review was posted on: 6 July 2020