Researchers assess effectiveness of contact tracing in controlling COVID-19 pandemic

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During the COVID-19 pandemic, Switzerland, like many other countries, relied on contact tracing to identify people who may have been contaminated by infected acquaintances. This strategy, which is ideal in epidemic outbreaks, has proven effective in preventing viral infections? Epidemiologists from the University of Geneva (UNIGE) and Geneva University Hospitals (HUG) analyzed data collected in Geneva. Overall, 40% of infected persons were identified through sick acquaintances. This rate fluctuates, however, depending on the variables involved, the type of housing and the wealth of the neighborhood. These results, published in the journal Euro surveillance, suggest that contact monitoring alone is not sufficient to control specific epidemics. It needs to be supplemented by different measures that take into account the specific characteristics of each disease.

Contact tracing aims to identify people who have been in contact with an infected person so that they can be treated before they can further transmit the disease.

“The effectiveness of this strategy depends mainly on the characteristics of the disease in terms of symptoms, contagiousness and mode of transmission,” explains Delphine Courvoisier, assistant professor of medicine at the UNIGE Faculty of Medicine, epidemiologist at the HUG department. Quality of care and during the COVID-19 pandemic, assigned by HUG as head of the ‘data’ unit in the Cantonal Medical Service, who conducted this work. “For example, in the case of Ebola, where patients are contagious only after the onset of symptoms, or in the case of measles that is close to us, contact tracing has proven its effectiveness in cutting the chain of transmission.”

To assess the effectiveness of contact tracing for COVID-19, Delphine Courvoisier and her team analyzed data from more than 140,000 cases and 185,000 contacts recorded in the Geneva canton between June 2020 and March 2022.

Voluntary or involuntary non-declaration?

To determine the number of people identified by contact tracing, we first need to determine the number of people who have infected each other. To do this, we looked at how many people living at the same address tested positive for SARS-Cov2 within ten days.”

Denis Mongin, Research Fellow in the Department of Medicine at UNIGE Faculty of Medicine, HUG’s Statistician and Specialist in Data Processing

“Then, to remove the element of chance, we ran a relocation test by randomly assigning people an address. The difference between the number of people who tested positive at the same address in a ten-day interval before and after the relocation indicated the number. The number of infected people at home, which were then declared contacts. are compared with those who do. Thus, we are able to estimate the overall rate of reporting contacts as well as its evolution over time and its dependence on the socio-economic profile of neighborhoods, building types and population density.”

On average, about 40% of infected people can be traced through contact tracing, varying from 25% at the height of epidemic waves to 60% during quiet periods. Socio-economic factors also play a major role. For example, the larger the building and the more communal the area (for example ground floor shops), the more people were likely to not report their contact. “This is probably due to inadvertent release: people bump into each other without thinking about it, they don’t necessarily know their neighbors, or the virus is suspended in the air, for example in the elevator,” explains Dennis Mongin. “What’s more, this effect disappears during the stages of restricted assembly and obligatory masking, which enables us to assess the effectiveness of these measures.”

On the other hand, the higher the socio-economic status of the neighborhood, the fewer people reported their contacts. “Many hypotheses have been put forward: less compliance with government directives, but a greater possibility of self-isolation due to the size of the housing and occupation that allows people to work from home without the need for a medical certificate,” said Delphine Courvoisier “In any case, it is their Demonstrates the importance of involving sociologists and anthropologists in the development and evaluation of health policies to understand the human factors involved in success or failure.”

One measure among many

Covid-19 is a highly contagious disease, transmitted by aerosol and contagious before symptoms begin. These features make contact tracing particularly complicated. In light of these results, was this strategy the best solution for reducing the transmission chain? “Contact tracing alone had a relative impact on the dynamics of the epidemic. But its importance as a psychological support for the population should not be overlooked, to reassure and listen to people during these anxiety-inducing periods. The idea is not to rewrite history and question the decisions that made sense at the time, but to use these experiences to build a more robust, multimodal response when we face another large-scale pandemic,” the authors conclude.


Journal Reference:

Mongin, D., etc. (2024). Time trends and modifiable factors in COVID-19 contact tracing coverage, Geneva, Switzerland, June 2020 to February 2022. Euro surveillance.

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