No discernable difference in SARS-CoV-2 viral loads of young children versus older children and adults

It is essential to understand the role of children in the transmission of the virus during the COVID-19 pandemic period. Notably, it is found there is no difference in the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) viral loads in young children and older children or adults when positive cases were tested for viral genes using RT-PCR.

This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like. Image captured and colorized at NIAID
This transmission electron microscope image shows SARS-CoV-2, the virus that causes COVID-19, isolated from a patient in the U.S. Virus particles are shown emerging from the surface of cells cultured in the lab. The spikes on the outer edge of the virus particles give coronaviruses their name, crown-like. Image captured and colorized at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Montana. Credit: NIAID

The coronavirus disease 2019 (COVID-19) pandemic is still prevalent worldwide, and the role of children in spreading the virus is still not exact. In most studies, children are underrepresented – ‘making their contribution to viral transmission elusive.’ It will throw light on how societies may function with children, especially when considering the safe reopening of schools. Sharline Madera et al., in a recent medRxiv* preprint paper, show a detailed investigation of viral nucleic acid among young children and adults, involving over 5,000 cases confirmed by RT-PCR (Reverse Transcriptase – Polymerase Chain Reaction) assay.

Age distributed nasopharyngeal SARS-CoV2 viral nucleic acid content. SARS-CoV2 viral nucleic acid detected by real-time RT-PCR in nasopharyngeal swabs from patients infected with SARS-CoV-2
Age distributed nasopharyngeal SARS-CoV2 viral nucleic acid content. SARS-CoV2 viral nucleic acid detected by real-time RT-PCR in nasopharyngeal swabs from patients infected with SARS-CoV-2

They recorded the cycle threshold value (Ct) for each positive sample. Ct value indicates the number of RT-PCR cycles needed for a detectable amplification of the nucleic acid. A high Ct value maybe because of low viral load; it can be suggestive of the amount of virus in an infected person.

The SARS-CoV-2 viral nucleic acid was detected by real-time RT-PCR, for both E and N genes. The CoV E protein has recently been shown to inhibit the host cell stress response, implicating it in pathogenesis; and the N gene codes for the viral nucleoprotein. For simplicity, they are showing only the E gene Ct values in this paper.

The authors report results based on testing done during March-August 2020. Nasopharyngeal swabs were collected from infected patients. Ct values across age groups were compared to ascertain potential differences in viral load. Samples were considered positive if the Ct value was ≤ 40, and otherwise, it was negative. No significant differences in Ct values were observed across the three groups: children aged less than five years, youth aged five to seventeen, and adults aged eighteen and older. Notably, the children did not show higher nasopharyngeal viral loads than others.

While a previous study observes a higher nasopharyngeal viral load in children below the age of 5 years, the undertaken study involves a population of asymptomatic and mildly symptomatic patients – most of whom are outpatients. Thus, the authors warn that their observations may apply primarily to the outpatient setting and not extend to the severely ill or hospitalized patients. It is also important to bear in mind that there are conflicting reports of studies where high viral loads are observed with no symptoms, and low viral loads are seen in hospitalized patients.

The viral load changes rapidly since the onset of infection and appearance of symptoms. Thus, the sampling time is a significant variable.

The data used in this study comes from a robust sample size of 5444 patients - especially about 200 children, aged less than 5 years - can be a likely representative of the general population of infected children and others.

When using this data, the authors caution that the PCR amplification also occurs from infectious virions, defective viral particles, or lysed infected cells. Infectivity in any population is also affected by other clinical, behavioral, and environmental factors. The authors, however, argue that children are not highly infectious due to higher viral loads.

"Our findings argue against the idea that young children are more infectious due to higher viral loads, and suggest an alternative explanation for their contribution to SARS-CoV-2 transmission, such as representing a reservoir of asymptomatic infections. Ultimately, the role of  young children in the spread of SARS-CoV2 can only be determined by careful clinical and epidemiological studies that examine transmission events in the population."

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
  • Nasopharyngeal SARS-CoV2 viral loads in young children do not differ significantly from those in older children and adults, Sharline Madera, Emily Crawford, Charles Langelier, Nam K Tran, Ed Thornborrow, Steve Miller, Joseph L DeRisi, medRxiv 2020.09.17.20192245; doi: https://doi.org/10.1101/2020.09.17.20192245
Dr. Ramya Dwivedi

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Dr. Ramya Dwivedi

Ramya has a Ph.D. in Biotechnology from the National Chemical Laboratories (CSIR-NCL), in Pune. Her work consisted of functionalizing nanoparticles with different molecules of biological interest, studying the reaction system and establishing useful applications.

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