Length of exposure time to COVID-19 ‘a major risk for transmission’

The NHS COVID-19 contact tracing app successfully predicted people’s risk of catching the virus following exposure to an infected person, according to a new study.

The University of Oxford’s Pandemic Sciences Institute, UK, analysed the app’s exposure measurements for seven million people who were notified by the app that they had come into contact with the virus and found the app correctly predicted people’s risk of infection based on these measurements.

Writing in the latest edition of Nature, the team say the data provide new insight into how COVID-19 spreads.

Although being closer to those with COVID-19, and spending more time close to them, is known to increase the risk of someone catching the virus, this is the first large-scale study to reveal how much, and how these two factors act together.

Lead analyst Dr Luca Ferretti, research fellow at the Pandemic Sciences Institute, said: “This study found that how long you are exposed to someone infected with COVID-19 is the biggest factor affecting whether or not you’ll become infected yourself.

“Many infections resulted from long exposures. The risk of infections keeps increasing for every hour spent in close proximity. Infection is not inevitable after a short exposure, rather the risk keeps building up as the hours go by.

“18 million people in England and Wales used the NHS COVID-19 app, and thanks to them, millions of cases and over 10,000 deaths were averted.”

The team says these findings have implications for other respiratory infections, including the design of measures to prepare for a future pandemic.

The study’s principal investigator Professor Christophe Fraser, of the Pandemic Sciences Institute, said: “These findings show how good data could be used in a future outbreak to inform social distancing precautions. In the case of COVID-19, this might have led to more emphasis on duration of contact as a risk for transmission.

“As governments and the global health community consider measures to address how to manage future epidemics and pandemics, improving our tools to understand how viruses spread is crucial. Risk management decisions need to be firmly based on quantitative evidence such as this.”

Digital measurement of SARS-CoV-2 transmission risk from 7 million contacts Nature 20 December 2023



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