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At the University of Oxford, the researchers revealed the surprising relationship between exposure time and infection risk during COVID-19 contact. Their findings suggest that the duration of exposure to COVID-19 has a more significant impact on the risk of infection compared to proximity.
Their study, titled “Digital measurement of SARS-CoV-2 transmission risk from 7 million contacts,” is featured in Nature. The Oxford team extensively analyzed the data from the NHS COVID-19 app in England and Wales, seeking to understand the likelihood of SARS-CoV-2 transmission after exposure.
They closely examined 7 million COVID-19 contacts recorded by the app, spanning from April 2021 to February 2022 and involving 23 million hours of exposure. Out of these, 240,000 tests reported positive results. The app evaluated the contacts based on proximity, duration, and infectiousness scores. These scores were then used to determine the risk of transmission.
Analyses conducted using the privacy-preserving Exposure Notification framework utilized Bluetooth signal strength measurements to estimate proximity between smartphones.
The study revealed that closer proximity resulted in more rapid exposure scores. Duration was evaluated in 30-minute blocks during which COVID-19 contact proximity was observed.
The data indicated that shorter and lower-risk encounters were more common, whereas transmissions occurred across a broad spectrum of risk levels, varying in duration from a few hours to several days. Notably, household and recurring contacts, despite being a smaller percentage of app-recorded COVID-19 contacts, were responsible for more transmissions due to the longer durations and closer proximity experienced in those settings.
It was found that the duration of exposure significantly influenced the transmission of the virus. Short exposures demonstrated a linear increase in the probability of reported transmission at a rate of 1.1% per hour. However, after a few hours, the rate of increase slowed down, indicating that the likelihood of infection continued to rise.
Though the app data provided valuable insights, the study acknowledged some limitations. Notably, the data may be biased, as it is primarily generated by users who are cautious enough to use a risk and traceability app. Additionally, self-reporting of COVID-19 positive test results, which is required for the app’s data, may not be completed by all users, thus potentially hindering the risk calculation.
Despite these limitations, the study’s analysis provided a rare opportunity to observe millions of interactions, offering the best available insights into the risks associated with COVID-19 exposure. The research validated the app’s risk score calculations, emphasizing the strong correlation between app-recorded measurements (proximity, duration) and the actual probability of reported transmissions.
For more information, the study “Digital measurement of SARS-CoV-2 transmission risk from 7 million contacts” can be found here. You can also Read More.
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