Post-COVID-19 multimorbidity incidence by prior vaccination status in people with a pre-existing comorbidity: A population-based cohort study.

Background: Long-term health consequences of COVID-19, particularly among individuals with pre-existing chronic diseases, are not fully understood. This study investigates whether SARS-CoV-2 infection increases the risk of developing multimorbidity (≥2 chronic conditions) and evaluates protective effects of vaccination.

Methods: We analyzed territory-wide electronic health records from Hong Kong, linking Hospital Authority data with COVID-19 infection and vaccination records from the Department of Health. A retrospective matched-cohort study was conducted among patients with one pre-existing chronic condition. Participants were stratified into three groups: (1) no documented COVID-19 infection, (2) COVID-19 infection with incomplete vaccination (<3 doses), and (3) COVID-19 infection with full vaccination (≥3 doses). The primary outcome was the incidence of a second chronic condition from a pre-specified list.

Results: Among 1,038,175 eligible individuals, 68,975 (6.64%) developed multimorbidity over a median follow-up of 192 days (IQR: 96-313). The non-COVID-19 group (51,288 cases) had an incidence rate of 68.88 per 1000 person-years (95% CI: 68.18-69.37). In contrast, the COVID-19/unvaccinated group (9455 cases) exhibited a significantly higher rate (86.58; 95% CI: 84.85-88.35). The COVID-19/vaccinated group (8232 cases) showed a moderated rate (72.84; 95% CI: 71.27-74.43). Adjusted incidence rate ratios were 1.26 (95% CI: 1.23-1.29) for unvaccinated and 1.08 (95% CI: 1.05-1.11) for vaccinated individuals compared to the non-COVID-19 group. Results remained consistent across age, sex, and comorbidity subgroups.

Interpretation: COVID-19 infection is associated with an increased risk of multimorbidity in patients with pre-existing conditions. Full vaccination attenuates this risk substantially, highlighting its critical role in mitigating post-infection complications.