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Asymptomatic COVID-19 individuals carry similar viral loads as symptomatic patients

Pearl Toh
20 Nov 2020

Individuals positive for COVID-19 who are asymptomatic carry similar viral loads as symptomatic patients, according to a study on a population in nursing homes presented during IDWeek 2020 — underscoring the importance of personal protective equipment (PPE) to limit viral transmission.

“Up to half of transmission comes from people without symptoms,” said presenting author Dr Roby Bhattacharyya from Massachusetts General Hospital in Boston, Massachusetts, US. “Transmission of COVID-19 from people without symptoms confounds public health containment strategies.”

Given the wide spectrum of disease presentation of COVID-19 — ranging from asymptomatic to fatal cases — the investigators were keen to find out what causes the variability in COVID-19, and whether viral load can explain some of the variability.  

“Is nasopharyngeal viral load of SARS-CoV-2 related to symptom status at the time of testing?” Bhattacharyya posed the question during his presentation.  

Answers to this question hold implications for transmission and the role of asymptomatic screening, he pointed out.

In the cohort study,15,514 staff (mean age 45 years, 76 percent female) and 16,966 residents (mean age 82 years, 65 percent female) at nursing homes and assisted living facilities underwent screening by RT-PCR of nasopharyngeal swab samples. [IDWeek 2020, abstract LB-11]

Overall, 2654 residents (15.5 percent) and 624 staff (4.1 percent) turned out positive for SARS-CoV-2.

Among the participants who tested positive, a majority were asymptomatic at the time of testing, including 70.8 percent of infected residents and 92.4 percent of infected staff.

“These findings highlight a role for testing individuals even though they are asymptomatic, at least when incidence is high,” said Bhattacharyya.

When analysed by symptom status, the prevalence of positivity was 12.7 percent and 3.7 percent for asymptomatic residents and staff, respectively, compared with 53.1 percent and 18.2 percent, respectively among those who showed symptoms.

Although the range of Ct values of RT-qPCR were spanning wide, the distribution was similar between patients with symptoms and those without — indicating nasopharyngeal viral load was unrelated to symptom status at the time of testing.

“In a large cohort of individuals screened for SARS-CoV-2 by RT-qPCR, we found strikingly similar viral load distributions in patients with or without symptoms at the time of testing during the local peak of pandemic,” observed Bhattacharyya.

Furthermore, this finding persisted across all prespecified subgroups examined, including age, sex, race and ethnicity.

“Because the distributions of viral loads in infected individuals irrespective of symptomatology are very similar, existing testing modalities that have been validated for detection of SARS-CoV-2 RNA in symptomatic patients should perform similarly in individuals without symptoms at the time of testing,” Bhattacharyya suggested.

Nonetheless, as there was no longitudinal follow-up, he pointed out that whether viral load was related to disease severity in later trajectory of the disease course could not be ascertained.

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