Fig 2. Rule-Based PCT Overview.

This simplified diagram shows how information flows through the rule-based PCT algorithm. This algorithm is run each day on the agent’s 14-day history of features to estimate their risk history. Each day the algorithm is run, it takes as input their current RT-PCR test history, user-input symptom history, and anonymized risk message history. Next, a ruleset is applied independently to each input type yielding estimates of the user’s risk history over the past 14 days. Finally, to construct a single conservative estimate of the user’s risk history, RB-PCT takes the maximum risk across all estimates including the previously generated estimates. There are some exceptions to these rules (e.g. negative test results reset some of the risk history to low values); a full description is provided in S3 Appendix.