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SARS-CoV-2 viral loads in infected patients

Coronavirus disease 2019 (COVID-19), caused by the SARS-CoV-2 virus, was first detected in Wuhan, China in December 2019. Since that time, the World Health Organization has declared COVID-19 a public health emergency due to its rapid spread.

A recent study published in The New England Journal of Medicine assessed SARS-CoV-2 viral loads in patients to try to better understand the transmission of the virus.

There were a total number of 18 patients in this study, made up of an equal number of men and women. The age range was 26-76, with a median age of 59 years old. Researchers analyzed nasal and throat swabs that were taken from 14 patients who had recently been to Wuhan, China. Swabs were taken from four patients who were considered secondary cases. There were three patients who needed intensive care. However, most cases were considered to be mild-to-moderate.

On January 11th, two Wuhan residents – whose symptoms had recently commenced – visited their daughter in Zhuhai. Their daughter experienced a fever six days after her parents’ visit, making her one of the four secondary cases. Researchers found viral ribonucleic acid (RNA) on swabs taken on the first day her symptoms began.

On January 17th, one patient had visited his friend, mother, and wife (the remainder of the secondary cases). The patients’ wife and mother started experiencing symptoms 3-5 days after his visit. Viral RNA was visible shortly after the women’s symptoms commenced. Furthermore, the patient’s friend was asymptomatic but tested positive for COVID-19 seven, ten, and 11 days after contact.

All 17 patients who experienced symptoms were tested on the day of symptom onset.  The study reported that shortly after symptom onset, high viral loads were present. There were lower viral loads in the throat than the nose. The researchers noted that the shedding of SARS-CoV-2 viral RNA seems similar to that seen with Influenza, but fairly different to SARS-CoV.

This study highlights that in both symptomatic and asymptomatic patients, the transmission of COVID-19 may transpire in the earlier days of infection. Due to this, to efficiently detect and isolate persons infected with COVID-19, different approaches than those applied throughout the SARS outbreak are likely needed. It is believed that advanced testing and data transmission are needed to help contain the spread of COVID-19. Future research on SARS-CoV-2 viral loads is suggested to help further understand its transmission. 

Written by Laura Laroche, HBASc, Medical Writer


Zou, Lirong, MSc, et al. “SARS-CoV-2 Viral Load in Upper Respiratory Specimens of Infected Patients.” The New England Journal of Medicine. March 19th, 2020.

Image by Ewa Urban from Pixabay 



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