‘Viral load’ is main determinant of Covid-19 transmission risk: Study

Prashasti Awasthi Mumbai | Updated on February 05, 2021

‘Increased viral loads among asymptomatic contacts are also strongly associated with higher risk of developing symptomatic Covid-19’

A new study carried out in the United Kingdom and Spain has revealed that viral load is the main determinant of the transmission risk of Covid-19.

The study, published in the journal Lancet Infectious Diseases, noted that increased viral loads among asymptomatic contacts were also strongly associated with a higher risk of developing symptomatic Covid-19. This association was further reflected in the incubation time, which became shorter with increasing viral loads.

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The team wrote: “When a patient with high viral load is identified, the implementation of reinforced contact tracing measures and quarantines might be crucial to reduce onward transmission.”

The virus has been detected in respiratory tract samples one to two days before the onset of the symptoms. It can persist for as long as several weeks post symptom onset.

However, the detection of viral RNA does not necessarily reflect infectiousness, and the precise relationship between viral load among cases and the risk of transmission to contacts is still not clear.

“Although studies have suggested that the viral load of cases might be associated with the risk of disease or transmission, no published data so far have directly addressed this question,” said the researchers.


For the study, the team conducted a post-hoc analysis of data collected during a randomised controlled trial of Covid-19 cases and their close contacts between March 17 and April 28, 2020.

All Covid-19 cases were aged 18 years or older, were not hospitalised, and had polymerase chain reaction (PCR) test results available at baseline.


The team identified 314 Covid-19 patients, of whom 282 (90 per cent), had at least one close contact and gave a total of 753 contacts.

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The viral load of the index case was the leading factor determining the risk of PCR-confirmed SARS-CoV-2 infection among contacts.

Notably, the multivariable analysis revealed no association between sex, age, diabetes, cardiovascular disease, or respiratory disease and the risk of or time to developing symptomatic Covid-19.

“Taken together, our results indicate that the viral load, rather than symptoms, might be the predominant driver of transmission,” writes Marks and colleagues.

Published on February 05, 2021

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