Effect of an enhanced public health contact tracing intervention on the secondary transmission of SARS-CoV-2 in educational settings: the four-way decomposition analysis

Abstract Background We assessed the impact of testing contacts immediately instead of at the end of quarantine on the transmission of SARS-CoV-2 in schools in Reggio Emilia Province. Methods We analysed surveillance data on notification of COVID-19 cases in schools between 1 September 2020 and 4 April 2021. Results Median tracing delay decreased from 7 to 3.1 days and the percentage of the known infection source increased from 34% to 54.8% (IRR 1.61 1.40-1.86). Implementation of prompt contact tracing was associated with a 10% decrease in the number of secondary cases (excess relative risk, EER -0.1 95%CI -0.35 to 0.15). Knowing the source of infection of the index case led to a decrease in secondary transmission (IRR 0.75 95% CI 0.63-0.91) while the decrease in tracing delay was associated with decreased risk of secondary cases (1/IRR 0.97 95%CI 0.94-1.01 per one day of delay). The direct effect of the intervention accounted for the 29% decrease in the number of secondary cases (EER -0.29 95% -0.61 to 0.03). Conclusions Prompt contact testing in the community seems to reduce the time of contact tracing and increases the ability to identify the source of infection in school outbreaks. Yet, observed differences can be also due to differences in the force of infection and to other control measures put in place. Funding This project was carried out with the technical and financial support of the Italian Ministry of Health – CCM 2020 and Ricerca Corrente Annual Program 2023.