In situations in which the user becomes a positive COVID-19 case, he/she can voluntarily share data captured within the past two weeks with the backend database management system. An AWS product Amazon Cognito was used to control user authentication and access to data storage. As shown in Figure 8c, users are required to sign in/up for an Amazon Cognito account in order to share their information. After signing in as an authorized client, users can publish their internal information to the Amazon cloud as shown in Figure 8d. All of the data related to the COVID-19 cases will be stored and managed in the DynamoDB database in the Amazon cloud. Our developed application was connected to the DynamoDB using another AWS product, the IoT Core. When new data is added to cloud storage, the contact tracing application will look for any matches between the backend data and the data stored internally in the user device. If it finds any matches that show that a confirmed COVID-19 positive case and the target user were close to each other for more than 15 min, the application will then notify the target user about potential exposure to COVID-19 and alert cleaning staff to disinfect the place. This process is shown in Figure 9. A demo of people trajectories is shown in Supplementary Materials.