A U.S. study from the city of Detroit, presented at this week's ESCMID Conference on Coronavirus Disease (ECCMID, online 23-25 September) shows that the initial SARS-CoV-2 viral load in nasopharyngeal samples has been decreasing as the pandemic progressed. The authors also observed that the decline in viral load was associated with a decrease in death rate. The study is by Dr. Said El Zein, Wayne State University/ Detroit Medical Center, Detroit, and colleagues.

The dynamics of the SARS-CoV-2 viral load (VL) on a population level remain poorly characterized. In this study, the authors present data describing the downward trend in the initial SARS-CoV-2 VL in nasopharyngeal swab samples of hospitalized patients in Detroit, Michigan during the period of April 4 through June 5, 2020.

They conducted a retrospective study that included all hospitalized patients who had initial nasopharyngeal swab samples analyzed at the Detroit Medical Center, that returned positive for SARS-CoV-2 by PCR testing between April 4 and June 5, 2020. To estimate the viral load, the authors used the so-called cycle threshold (Ct) value provided by the test for each sample - a higher Ct indicates a lower viral load. Based on their studies, the authors designated high, intermediate, and low VL samples to have a Ct value of 25 or under, 26-36, and 37 or over, respectively.

During the first week of the study (week of April 4, 2020), 49% of the initial VL samples were in the intermediate group, compared to 25.5% each in the low and high VL categories respectively. Thereafter, there was a progressive decline in the percentage of samples in the high and intermediate VL categories with a concomitant rise in the percentage of samples in the low VL category.

By week five of the study, 70% of the positive samples had an initial low VL. This trend in initial VL coincided with a decrease in the percent of deaths. Almost half of the patients in the high VL group died (45%) compared to 32% and 14 % of the intermediate and low VL categories respectively.

The authors conclude, "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.AltThough confounding variables have not been evaluated, this suggests an association between initial viral load and mortality."

El Zein adds, "Exact reasons for a decrease in initial viral load over time are unclear. 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."