From the Journals

Among asymptomatic, 2% may harbor 90% of community’s viral load: Study


About 2% of asymptomatic college students carried 90% of COVID-19 viral load levels on a Colorado campus last year, new research reveals. Furthermore, the viral loads in these students were as elevated as those seen in hospitalized patients.

“College campuses were one of the few places where people without any symptoms or suspicions of exposure were being screened for the virus. This allowed us to make some powerful comparisons between symptomatic vs healthy carriers of the virus,” senior study author Sara Sawyer, PhD, professor of virology at the University of Colorado, Boulder, said in an interview.

“It turns out, walking around a college campus can be as dangerous as walking through a COVID ward in the hospital, in that you will experience these viral ‘super carriers’ equally in both settings,” she said.

“This is an important study in advancing our understanding of how SARS-CoV-2 is distributed in the population,” Thomas Giordano, MD, MPH, professor and section chief of infectious diseases at Baylor College of Medicine, Houston, said in an interview.

The study “adds to the evidence that viral load is not too tightly correlated with symptoms.” In fact, Dr. Giordano added, “this study suggests viral load is not at all correlated with symptoms.”

Viral load may not be correlated with transmissibility either, said Raphael Viscidi, MD, when asked to comment. “This is not a transmissibility study. They did not show that viral load is the factor related to transmission.”

“It’s true that 2% of the population they studied carried 90% of the virus, but it does not establish any biological importance to that 2%,” added Dr. Viscidi, professor of pediatrics and oncology at Johns Hopkins University, Baltimore,.

The 2% could just be the upper tail end of a normal bell-shaped distribution curve, Dr. Viscidi said, or there could be something biologically unique about that group. But the study does not make that distinction, he said.

The study was published online May 10, 2021, in PNAS, the official journal of the National Academy of Sciences.

A similar picture in hospitalized patients

Out of more than 72,500 saliva samples taken during COVID-19 screening at the University of Colorado Boulder between Aug. 27 and Dec. 11, 2020, 1,405 were positive for SARS-CoV-2.

The investigators also compared viral loads from students with those of hospitalized patients based on published data. They found the distribution of viral loads between these groups “indistinguishable.”

“Strikingly, these datasets demonstrate dramatic differences in viral levels between individuals, with a very small minority of the infected individuals harboring the vast majority of the infectious virions,” the researchers wrote. The comparison “really represents two extremes: One group is mostly hospitalized, while the other group represents a mostly young and healthy (but infected) college population.”

“It would be interesting to adjust public health recommendations based on a person’s viral load,” Dr. Giordano said. “One could speculate that a person with a very high viral load could be isolated longer or more thoroughly, while someone with a very low viral load could be minimally isolated.

“This is speculation, and more data are needed to test this concept,” he added. Also, quantitative viral load testing would need to be standardized before it could be used to guide such decision-making


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