Methodological details:

• Surveillance platform

A total of 10,000 participants recruited from all 18 districts around Hong Kong were provided with rapid antigen tests (RAT).

• Daily testing data

Participants are grouped into seven cohorts of roughly equal size, with each cohort scheduled to perform RAT on one day over the week. Participants conducted the RATs with self-sampled combined throat and nasal swab, and submitted their test results on the same day.

• Daily point prevalence

Point prevalence is estimated by the number of individuals reported positive results, divided by the number of individuals submitted valid test results on the same day. It reflects the estimated proportion of the population that has COVID-19 infection on a particular day, including newly occurring cases and existing cases that have not yet recovered. The estimate is age-standardised, a weighted average of the age-specific point prevalence where the age-specific weights represent the relative age distribution of the population structure in Hong Kong according to the 2021 Population Census.

• Confidence intervals for point prevalence

Confidence intervals are calculated using the Fay-and-Feuer method based on gamma distribution (1997) to show the uncertainty around the estimate. Wider intervals suggest greater uncertainty in the estimate.

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