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An analysis of SARS-CoV-2 viral load by patient age

Our take —

This is an important non-peer reviewed study with important flaws. The question of whether children infected with SARS-CoV-2 can transmit the infection as readily as adults is crucial to decisions about school closures and societal re-opening. Although the authors report no differences in viral loads across age brackets, the statistical approach they used was not the ideal way to detect differences by age. Moreover, they did not report important details about the individuals providing samples that may have affected the viral loads, including disease severity and the timing of testing relative to symptom onset. Even with the data added, our understanding remains too poor to reach any definitive conclusions about the transmissibility of SARS-CoV-2 from children.

Study design


Study population and setting

This study included 3,712 patients who tested positive for SARS-CoV-2 infection via real-time PCR in a laboratory network in Berlin, Germany from January to April 26, 2020. Viral loads were estimated from throat swab samples and projected per mL of sputum or per entire swab sample. Patients were categorized into age brackets in two ways: by decade (e.g., 1-10 years, 11-20 years, etc.) and by “social stratum” (0-6 years, 7-11 years, 12-19 years, 20-25 years, 26-45 years, >45 years).

Summary of Main Findings

Median log10 viral loads (e.g. 4.64=10^4.64) per sample by decade of age were as follows: 0-10 years: 4.64, 11-20 years: 4.80, 21-30 years: 5.26, 31-40 years: 5.21, 41-50 years: 4.99, 51-60 years: 5.26, 61-70 years : 5.28, 71-80 years: 5.17, 81-90 years: 5.34, 91-100 years: 5.61. There was evidence for at least one pairwise difference in viral load distributions by age group. However, with an increased threshold for significance after adjusting for multiple tests, no single pairwise comparison of viral load distributions between age groups was statistically significant.

Study Strengths

The sample size was reasonably large.


The authors did not treat age as a continuous variable, which would have maximized power to detect differences in viral load and allowed for a more thorough assessment. There is no information on the selection of samples, including disease severity, symptom profile, or stage of illness, all of which may contribute to measured viral load. The distribution of viral loads among younger ages appears to be lower than among adults, and a nonparametric test provided strong evidence of at least one difference between groups. Given these facts, and the small number of individuals in the youngest age groups, it should not be concluded that viral loads are equivalent across age brackets. It should also be noted that an independent re-analysis of these same data found a significant increase in viral loads by age (, even if viral loads are similar between age groups, this does not mean that transmissibility of the virus is equivalent. Further study would be needed to draw such a conclusion.

Value added

This is among the first studies to examine age-specific viral loads in SARS-CoV-2 infection.