Time trends and modifiable factors of contact tracing coverage in Geneva, Switzerland, June 2020 to February 2022

Background Contact tracing has been one of the central non-pharmaceutical interventions implemented worldwide to try to control the spread of Sars-CoV-2, but its effectiveness strongly depends on its ability to detect contacts. Methods We analysed 166’892 concomitant infections occurring at the same address from June 2020 until February 2022 using an extensive operational database of SARS-CoV-2 tests in Geneva and used permutations statistics to compare the total number of secondary infections occurring at the address with those reported through contact tracing. Results Manual contact tracing captured on average 41% of the secondary infections, with variation in time from 23% during epidemic peaks to 60% during low epidemic activity. People living in wealthy neighbourhoods were less likely to report contacts (adjusted odds ratio (aOR): 1.6). People living in buildings, compared to people living in single house, were also less likely to report contacts than those living in houses, with an aOR of 1.1 to 3.1 depending on the variant, the size of the building and the presence of shops. This under-reporting of contacts in buildings decreased during periods of mandatory face masking and restriction of private gathering. Conclusions Contact tracing alone does not detect enough secondary infections to efficiently reduce the propagation of Sars-CoV-2. Public messages and outreach campaigns targeting specific populations, such as those in affluent areas, could enhance coverage. Additionally, measures like wearing face masks, improving ventilation, and implementing gathering restrictions should also be considered to reduce the number of infections occurring during interactions that may not be perceived as high risk.