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.
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.
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).
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