Study: Initial SARS-CoV-2 Viral Loads Decreased As Pandemic Progressed


Investigators also reported a potential association between initial viral load and mortality in patients with COVID-19.


Results of a study based in Detroit, Michigan reported that the initial viral load (VL) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in nasopharyngeal samples has been decreasing throughout the pandemic’s progression.

The study was presented at the ESCMID Conference on Coronavirus Disease, held online September 23-25, 2020.

According to the investigators from Wayne State University/Detroit Medical Center, the VL decline was also associated with a decrease in death rate. The study took place between April 4 and June 5, 2020 among hospitalized patients in Detroit.

"During the April-June 2020 period of the COVID-19 pandemic, the initial SARS-CoV-2 load steadily declined among hospitalized patients with a corresponding decrease in the percent of deaths over time,” the authors concluded. “Though confounding variables have not been evaluated, this suggests an association between initial viral load and mortality."

However, the dynamics of SARS-CoV-2 VL on a population level remain unclear. Study investigators focused on the initial SARS-CoV-2 VL in nasopharyngeal swab samples that returned positive for SARS-CoV-2 and utilized the cycle threshold (Ct) value provided by the test for each sample, in which a higher Ct demonstrates a lower VL.

Study authors labeled high, intermediate, and low VL samples with Ct values of 25 or lower, 26-36, or 37 or higher, respectively. In the first week of the study, 49% of the initial VL samples spanned the intermediate group; 25.5% VL samples were in the low and high VL categories. Following the first week, data demonstrated a progressive decline in the high and intermediate VL categories and a rise in the numbers of samples in the low VL category.

At week 5 of the study, 70% of samples showed an initial low VL and corresponded with a decrease in deaths; 45% of patients with high VL died, compared with 32% of those in the intermediate group, and 14% of patients in the low VL group.

"Exact reasons for a decrease in initial viral load over time are unclear,” study investigator Said El Zein, MD, Wayne State University/Detroit Medical Center, said in the press release. “A downward trend in the initial VL may reflect a reduction in the severity of the pandemic and trends in the viral load values over time may represent a marker to assess the progress of the pandemic. Rapid implementation of social distancing measures, lockdown, and widespread use of facemasks may have contributed to a decrease in the exposure to the virus."


  1. US study shows decline in viral load of patients with COVID-19 as pandemic progressed. News Release. EurekAlert; September 23, 2020. Accessed September 24, 2020.

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