# What Went Wrong? Predictors of Contact Tracing Adoption in Italy during COVID-19 Pandemic
## 1 Introduction  
Since the beginning of the pandemic, governments around the world identified non-pharmaceutical interventions (NPI) as an effective way to contain coronavirus disease (COVID-19 [1, 2]). Evidence of early pre-symptomatic disease transmission [3, 4] has questioned traditional containment measures based on symptom surveillance. For this reason, digital contact tracing (CT) apps have been selected globally as non-pharmaceutical tools to contain the COVID-19 contagion by exploiting the widespread use of smartphones. The main advantage of contact tracing methods is the possibility to immediately alert recent close contacts of an infected person, in order to push them towards self-isolation [5]. Many nations (e.g., Singapore, South Korea, Taiwan, Hong Kong, China) promptly adopted CT at the beginning of the pandemic [6, 7, 8], mainly due to the fact that they have already used this type of system in other outbreaks (including HIV, syphilis, and swine flu; [9]). For instance, Singapore was one of the first nations to introduce digital tools for tracing COVID-19 (GoS, TraceTogether, url: [[https://www.tracetogether.gov.sg/](https://www.tracetogether.gov.sg/)]([https://www.tracetogether.gov.sg/](https://www.tracetogether.gov.sg/)) (accessed on 20 May 2021)). Until May 2021, 37 countries relied on or used CT apps to deal with COVID-19 with different degrees of adoption by population (Figure 1) [10, 11]. As reported in Figure 1, CT systems have been particularly adopted in countries such as Finland, Ireland, Germany, UK, Switzerland, and Iceland. Notably, the countries that had the highest level of adoption were Singapore (92%) and New Zealand (55%). In a preliminary way, we exploratively compared the adoption of contact tracing systems with the percentage of infections in each country that adopted such a solution (if data were available) if the contagion of the population was \(\geq\)4% (see Table 1 (Appendix A), data updated to May 2021). This threshold, although chosen arbitrarily, has served to distinguish those countries in which the infection was more severe from those in which the infection was lower, since it is precisely in the former countries that the need to trace the infectionsemerges. Meanwhile, in areas where the contagion is somehow low, applying CT may not be so essential. The plotting of the 19/37 eligible countries' data (Figure 1) suggested that higher adoption of contact tracing systems was associated with a lower number of infections. Therefore, the success of the CT systems seems to depend on the adoption of the population [12]. For example, Italy, one of the countries most affected by the pandemic, had an unsatisfying adoption of the Italian CT system (i.e., Immuni; see Table 1 and Figure 1). The Immuni app was promoted by the Italian Health Ministry and made by the Bending Spoons society. Bending Spoons has granted the Italian Government a perpetual and irrevocable license on Immuni free of charge. Immuni was released in June 2020, three months after the official beginning of the COVID-19 pandemic, without any mandatory requirements, and just 17.3% of the Italian population had downloaded it by May 2021 (Table 1). The modest adoption of Immuni may have contributed to the spread of infections and, consequently, the adoption of more strict measures, such as the lockdown. The Immuni app is based on a technology called Bluetooth low energy (BLE). Once Immuni is installed on a device, it generates a temporary exposure key, which is random and modified every day to avoid identifiability and tracking issues for users. Based on this key, Immuni produces an identification code, which is then issued by the smartphone via BLE [13]. Immuni continuously saves the rolling proximity identifier of the nearby smartphones in its memory, and concurrently transmits its own identifier.