key: cord-0941722-zz8iki61 authors: Rannikko, Juha; Tamminen, Pekka; Hellsten, Roosa; Nuorti, J. Pekka; Syrjänen, Jaana title: Effectiveness of COVID-19 digital proximity tracing app in Finland date: 2022-03-11 journal: Clin Microbiol Infect DOI: 10.1016/j.cmi.2022.03.002 sha: 70c93858d169552eef1813f3576d2b2682994016 doc_id: 941722 cord_uid: zz8iki61 nan Mathematical modelling studies have suggested that digital proximity tracing (DPT) apps (also called digital contact tracing apps) have an impact on reducing COVID-19 transmission (1) (2) (3) . However, modelling studies have limitations that make extrapolation to real-world effectiveness difficult, and few studies with real-world data are available (4) (5) (6) (7) . Millions of people have been using the DPT apps, therefore their value in preventing onward transmission should be investigated in all COVID-19 incidence levels. The Finnish national DPT app "Koronavilkku" was launched in September 2020. It was downloaded by half of all mobile phone owners (2.5 million times) making it one the most penetrated national COVID-19 DPT apps (8) . "Koronavilkku" is produced by the Finnish Institute for Health and Welfare (THL) to alert the user that he/she may have been exposed to an individual with a positive laboratory result for COVID-19. Receipt of the alert and prevention of onward transmission requires several steps to occur: 1. Both the infected and the exposed individuals actively use the app. 2. Local authorities send a SMS activation code to the PCR test positive case. In our region, the code was not sent if the case had been in quarantine for 48h before symptom onset or the contact tracing (CT) call reveals that the app would not send meaningful notifications. 3. The Bluetooth signal has worked properly in the exposure setting. There are three parameters that are calculated: the distance between the mobile phones, the time spent in proximity, and the time since the index case's symptom onset. If a certain value is exceeded, the contact gets notified. 4 . The test positive case enters the activation code in the app within 4(-12) hours. Before the contacts receive the notification, there was a technical delay that was reduced from approx. 24h to 1-2h during the study period. 5. The potentially exposed person receives the notification before their infectious period begins, and changes behaviour accordingly. Due to strong privacy protection of the decentralised Finnish DPT system, the number of sent notifications is unknown; consequently, the attack rate among the recipients is unknown. We assessed the population-based effectiveness of the "Koronavilkku" app in the Pirkanmaa region (population 540,000) which had an estimated 250,000 DPT app users. Effectiveness was defined as the percentage of COVID-19 positive cases who had been notified by the app before being contacted by the CT team, and the percentage of positive cases who had changed their behaviour due to the notification. Data were obtained from the CT database of the Pirkanmaa Hospital District infectious diseases department that coordinates all COVID-19 CT in the region; all laboratory confirmed cases in the district are included in the database. All COVID-19 positive cases were interviewed by trained health care worker. This is a retrospective analysis of our contact tracing registry data; because of acute public health response, ethical review or informed consent was not required according to the Infectious Disease Law. From 1.10.2020 to 31.5.2021, 4557 PCR positive COVID-19 cases were identified; 4110 (90%) were ≥12 years old. The average 14-day case notification rate was 49 per 100,000 population. We were able to contact over 99% of the index cases; 99.4% within 24 hours. During the CT interview, we asked about "Koronavilkku" usage; of 2,368 cases (58% of cases ≥12 years) 970 (41%) reported actively using it. Based on information obtained during the CT call (see No 2 above) and occasional logistic problems, activation codes were sent to 541 (56%) cases. Activation codes were not sent to individuals whose use of the app was unknown. Based on information from the first CT call, the DPT app had given a notification before the call to 53 cases (2.2% of cases with usage information). These cases were interviewed again in a second call. Cases whose index was from the same household were excluded (N=15). Among the 38 cases, the exposure information was known for 20 cases before the app notification. For example, the case was in quarantine due to known exposure (other than household), the information had been disseminated in the workplace or a friend had told about the exposure. Of the remaining 18, only 8 (0.3% of cases with usage information; no data on 2 cases) reported having changed their behaviour because of the notification, for example, decided to get tested. The epidemiological impact of the NHS COVID-19 app Digital proximity tracing on empirical contact networks for pandemic control Quantifying SARS-CoV-2 transmission suggests epidemic control with digital contact tracing Effectiveness of contact tracing apps for SARS-CoV-2: a rapid systematic review Time to evaluate COVID-19 contact-tracing apps Early evidence of effectiveness of digital contact tracing for SARS-CoV-2 in Switzerland Epidemiological changes on the Isle of Wight after the launch of the NHS Test and Trace programme: a preliminary analysis Contact Tracing for Covid-19 -A Digital Inoculation against Future Pandemics