Neutralisation capacity against SARS-CoV-2 ancestral, Delta and Omicron variants before and after heterologous booster vaccination in Fijian health system workers.

Immunogenicity studies of booster doses of COVID-19 vaccines from Pacific Island countries are limited. A prospective cohort study was conducted amongst healthcare workers in Fiji to assess antibody neutralisation capacity against ancestral, Delta and Omicron (BA.2) variants following high force of infection from Delta during administration of primary dose schedules. Findings showed high baseline neutralisation prior to heterologous booster vaccinations and enduring protective neutralisation against all three variants of SARS-CoV2 at 6 months following booster receipt. Limited benefit from third doses in this population emphasises the need for pragmatic vaccination strategies guided by local disease contexts.