COVID-19 hospitalisations in immunocompromised individuals in the Omicron era: a population-based observational study using surveillance data in British Columbia, Canada

BACKGROUND: People with immune dysfunction are at higher risk of severe outcomes from COVID-19 infection, but relatively little epidemiologic information is available for mostly vaccinated population in the Omicron era. This population-based study compared relative risk of breakthrough COVID-19 hospitalisation among vaccinated people identified as clinically extremely vulnerable (CEV) vs non-CEV individuals before treatment became more widely available. METHODS: COVID-19 cases and hospitalisations reported to the British Columbia Centre for Disease Control (BCCDC) between January 7, 2022 and March 14, 2022 were linked with data on their vaccination and CEV status. Case hospitalisation rates were estimated across CEV status, age groups and vaccination status. For vaccinated individuals, risk ratios for breakthrough hospitalisations were calculated for CEV and non-CEV populations matched on sex, age group, region, and vaccination characteristics. FINDINGS: Among CEV individuals, a total of 5591 COVID-19 reported cases were included, among which 1153 were hospitalized. A third vaccine dose with mRNA vaccine offered additional protection against severe illness in both CEV and non-CEV individuals. However, 2- and 3-dose vaccinated CEV population still had a significantly higher relative risk of breakthrough COVID-19 hospitalisation compared with non-CEV individuals. INTERPRETATION: Vaccinated CEV population remains a higher risk group in the context of circulating Omicron variant and may benefit from additional booster doses and pharmacotherapy. FUNDING: BC Centre for Disease Control and Provincial Health Services Authority.