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Contact tracing during Phase I of the COVID-19 pandemic in the Province of Trento, Italy: key findings and recommendations

Our take —

This is a contact tracing study based in Trento, Italy, published as a preprint and thus not yet peer reviewed. There were 2,812 cases and 6,690 contacts. There was an overall secondary attack rate of 13.3%, with the highest secondary attack rate occurring among contacts over the age of 75 years. However, index cases who were between the age of 0-14 years had the highest percentage of contacts who became infected (22%). There was no routine testing of contacts, so most of them were identified as a case through being symptomatic; thus, there is likely an underrepresentation of the secondary attack rates. However, the finding that the youngest age group of index cases had the highest transmission among their contacts is helpful to note when policy makers are making decisions on reopening schools.

Study design

Cross-Sectional, Ecological

Study population and setting

The provincial agency for health services (APSS) in Trento, Italy conducted contact tracing from March to April, 2020 using a contact tracing website. Data on cases was provided by the central local health unit database while data on contacts of cases was collected by telephone interviews contact tracers from each local health district. A contact was defined as anyone who had contact with a confirmed or probable case within 48 hours prior or 14 days after symptom onset.

Summary of Main Findings

There were 2,812 reported cases, with almost half having up to three contacts each, for a total of 6,690 contacts (890 of whom developed symptoms). Prior to the lockdown on March 10, 2020, (consisting of shutting down schools, universities, and businesses except for grocery stores, pharmacies, and newsstands), the majority of contacts were non-cohabitating family or friends (~37%); however, after March 10, the majority of contacts became household contacts (67%). Ultimately, household contacts comprised 56% of all contacts and non-cohabitating family or friends comprised 27%. The secondary attack rate steadily increased with age (e.g. 18.9% among those 75 year and older vs. 8.4% among those 0-14 years). However, the youngest age group (0-14 years) were more likely to spread infection than any other age group, as 22% of their contacts became infected.

Study Strengths

This study has a large sample of cases and respective contacts. The contact tracing website also provides a centralized resource for data on cases and contacts that can be helpful for future analyses.

Limitations

Classifying a contact as a case was determinant on being symptomatic and having an epidemiological link as there was no routine testing conducted among contacts. Thus, the study is likely reporting an underrepresentation of how many contacts became infected (especially among younger age groups as these groups are more likely to exhibit mild to no symptoms).

Value added

As schools are opening up in the United States and other countries, the fact that secondary infection was more likely to occur in the youngest age group in this study suggests a potential for high levels of transmission both in schools and households if there are not protocols in place to reduce transmission while children are in school.

This review was posted on: 24 August 2020