Early Epidemiological Evidence of Public Health Value of WA Notify, a Smartphone-based Exposure Notification Tool: Modeling COVID-19 Cases Averted in Washington State

Background Secure and anonymous smartphone-based exposure notification tools are recently developed public health interventions that aim to reduce COVID-19 transmission and supplement traditional case investigation and contact tracing systems. We assessed the impact of Washington State's exposure notification tool, WA Notify, in mitigating the spread of COVID-19 during its first four months of implementation. Methods Due to the constraints of privacy-preservation and anonymized data, aggregate metrics and disparate data sources were utilized to estimate the number of COVID-19 cases averted based on a modelling approach adapted from Wymant et al (2021) using the following parameters: number of notifications generated; the probability that a notified individual goes on to become a case; expected fraction of transmissions preventable by strict quarantine after notification; actual adherence to quarantine; and expected size of the full transmission chain if a contact had not been notified. Results The model was run on a range of secondary attack rates (5.1%-13.706%) and quarantine effectiveness (53% and 64%). Assuming a 12.085% secondary attack rate and 53% quarantine effectiveness, the model shows that 6240 cases were averted statewide during the first four months of its implementation. Based on an estimated COVID-19 case fatality of 1.4%, WA Notify saved 30-120 lives during the study period. Conclusions These findings demonstrate the potential value of exposure notification tools as a novel public health intervention to help mitigate the spread of COVID-19 in the U.S. As new variants emerge and non-essential travel bans are lifted, exposure notification tools may continue to play a valuable role in limiting the spread of COVID-19.