Hospitalisation and mortality outcomes after second COVID-19 vaccination among Australians living with severe mental illness: Findings from a matched case-control study using national data linkage.

OBJECTIVE: To compare the proportion of Australians living with severe mental illness (target group) and the general population (comparator group) who received the first, second, and third doses of the COVID-19 vaccination between 22 February 2021 and 30 June 2022; examine the odds of any, non-psychiatric, and COVID-19-related hospitalisations within 12 months of the second vaccination; and examine the odds of all-cause and COVID-19-specific mortality within 12 months. METHODS: We used a retrospective 1:4 age- and sex-matched case-control design. Subgroup analysis of individuals aged ⩾65 years old was also undertaken. This study used de-identified, linked administrative data from the Australian Immunisation Register, Pharmaceutical Benefits Scheme, Medicare Benefits Schedule, National Death Index, Demography data, and hospital records. RESULTS: The cohort included 375,335 individuals living with severe mental illness and 1,373,231 comparators. Between 22 February 2021 and 30 June 2022, the proportion of target group members receiving the first, second, and third doses was significantly lower than the comparators, albeit still high, and with a shorter average time between vaccinations. Adjusted odds of hospitalisation in the target group within 12 months of the second vaccination were 14.0 (95% CI = 13.8-14.2) for any, 9.7 (95% CI = 9.6-9.8) for non-psychiatric, and 19.0 (95% CI = 17.7-20.3) for COVID-19-related hospitalisations. Adjusted odds of all-cause mortality were 3.1 (95% CI = 3.0-3.1) and 9.4 (95% CI = 7.7-11.5) for COVID-19-related mortality. CONCLUSIONS: People with severe mental illness had significantly poorer hospitalisation and mortality outcomes after their second vaccination, highlighting persistent gaps in access and equity of care for this vulnerable population. STUDY REGISTRATION: ACTRN12624000117516.