In the first 2 years of the COVID-19 pandemic, Hong Kong adopted strict public health and social measures to stop community transmission of SARS-CoV-2. These include border screening and control, isolation of cases and quarantine of their contacts and universal masking. During this period, attack rates in Hong Kong were among the lowest globally.

To estimate the seroprevalence of COVID-19 among healthcare workers (HCWs) in Hong Kong in 2020 and 2021.

We reviewed contact tracing data from the Hong Kong Department of Health to identify COVID-19 cases reported among HCWs. Between June 2020 and December 2021, we conducted a longitudinal cohort study to estimate the seroprevalence of COVID-19 among HCWs working in hospitals and clinics in Hong Kong during the first 2 years of the COVID-19 pandemic.

Overall seropositivity of COVID-19 by plaque reduction neutralisation test during the first (May–October 2020) and second round (November 2020–April 2021) of the study was 0% (95% CI 0.00% to 0.49%) and 0.52% (95% CI 0.14% to 1.33%). After COVID-19 vaccines were offered to HCWs in February 2021, seroprevalence by surrogate virus neutralisation assay among cohort participants who provided biannual blood samples rose to 68.7% (95% CI 65.9%, 71.3%) and 80.2% (95% CI 76.8%, 83.2%) in round 3 (May–October 2021) and the first 2 months of round 4 (November–December 2021).

Seroprevalence in Hong Kong HCWs in our study was low despite considerable exposure to confirmed COVID-19 cases in some study participants. However, the low rate of community transmission may have also contributed to the observed low seroprevalence among HCWs in our cohort.