2023-12-27 08:23:04
At the Gare de Lyon, in Paris, in August 2023. DELPHINE GOLDSZTEJN /PHOTOPQR /LE PARISIEN /MAXPPP
What is our probability of being infected with the SARS-CoV-2 virus after being exposed to an infected person? For the first time, a study answers precisely, thanks to the analysis of data from seven million “contacts” (people exposed to a confirmed case) notified in the United Kingdom, between April 2021 and February 2022, by an application of contact tracing (“contact tracing”). Where it appears that the duration of contact with an infected person is very important in the risk of effective contamination. An evidence ? It was still necessary to prove it, with supporting figures.
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Produced by a renowned team from the University of Oxford, This study was published in the journal Nature December 20. Luca Ferretti and his colleagues analyzed the data packets that each active smartphone application sent automatically and anonymously to a central server: duration and proximity of contacts, infectiousness of the infected person (time elapsed since their first symptoms). Were considered as « contact cases » people exposed to less than two meters and for a minimum of fifteen minutes. Then, using the common data in these packets, the researchers matched exposures to actual infections.
Unsurprisingly, the study confirms that the probability of being infected increases with the duration of contact, proximity to the infected person and their infectiousness. But the authors go further: they show that the duration of exposure is the most determining factor. The probability of transmission initially increased linearly with this duration (1.1% per hour) and then continued to increase over several days. Most exposures were short-lived (median of forty-two minutes), but transmissions generally resulted from exposures ranging from one hour to several days (median of six hours). “Eight out of ten transmissions resulted from exposure lasting more than an hour”summarizes Mahmoud Zureik, professor of epidemiology and public health at the University of Versailles-Saint-Quentin-en-Yvelines and director of Epi-Phare.
“Very rigorous work”
Furthermore, 40% of the people analyzed caught Covid-19 following household contact (over several days), while they represented only 6% of the contacts recorded. Conversely, fleeting contacts (less than thirty minutes) represent half of declared contacts, but very few transmissions (around 10%). Between these two extremes, 23% of notified contaminations were linked to prolonged contact with an infected person, “typically in a professional setting”, notes Mahmoud Zureik. And 25% were linked to a single day of contact with a confirmed case.
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