MONITORING THE TEMPORAL TRAJECTORY OF SARS-COV-2 IN HONG KONG - DAILY ANTIGEN RAPID TESTING SURVEILLANCE (DARTS) SYSTEM

INTRO: The number of PCR-confirmed cases may serve poorly as a surveillance data for a representative disease activity, due to its bias towards symptomatic cases or people having an identifiable risk exposure, and the high dependency on changing testing policy. We reported a large-scale ad-hoc community surveillance initiative, Daily Antigen Rapid Testing Surveillance (DARTS) System, using self-performed rapid antigen tests(RAT). METHODS: A representative cohort of 10000+ individuals was enrolled over the territory. Participants were divided into 7 sub-cohorts to achieve a rolling schedule with 1400+ individuals on a daily basis. Participant performed the RAT regularly irrespective of symptom or exposure history, with a self-sampled throat-and-nasal swab. RAT result and photo were reported on the same day of testing through an online platform. Daily point prevalence was disseminated on a real-time dashboard to inform the situation awareness(https://covid19.sph.hku.hk/dashboard). FINDINGS: Since its launch during the peak of the fifth wave in March 2022, the system has tracked the changing trajectory of different phases of the Omicron pandemic, including the rapidly subsiding daily prevalence from an initial high value of 12.7% (8.4-18.7) in early March to a baseline of 0.6% (0.2-1.4) in early April, maintained with a non-zero baseline (0.1-0.3%) over May, and subsequent stepwise increase to 0.5% (0.2-1.2) in June. The reproduction number increased from 0.66(0.63,0.70) to 1.23(1.14,1.33) from March to June, signifying the gradual increase of residual Omicron transmission. CONCLUSION: Our DARTS system has demonstrated the feasibility of a participatory surveillance system using self-performed RAT, and its utility as an ad-hoc surveillance to timely reflect the rapidly changing epidemic trajectory. Regular testing irrespective of symptom and exposure risk helps to give more representative picture, including subclinical cases who also carried an implication of disease transmission. The use of RAT also helps to avoid the constraint of manpower and testing capacity, and has been quickly adopted for case definition.