Background: Current evidence suggests that older people and people with underlying comorbidities are at increased risk of severe disease MESHD and death MESHD following hospitalisation with COVID-19. As comorbidity increases with age TRANS, it is necessary to understand the age TRANS-adjusted relationship between comorbidity and COVID-19 outcomes, in order to enhance planning capabilities and our understanding of COVID-19. Methods: We conducted a rapid, comprehensive review of the literature up to 10 April 2020, to assess the international empirical evidence on the association between comorbidities and severe or critical care outcomes of COVID-19, after accounting for age TRANS, among hospitalised patients with COVID-19. Results: After screening 579 studies, we identified seven studies eligible for inclusion and these were synthesised narratively. All were from China. The emerging evidence base mostly indicates that after adjustment for age TRANS (and in some cases other potential confounders), obesity MESHD obesity HP, hypertension MESHD hypertension HP, diabetes mellitus MESHD diabetes mellitus HP, chronic obstructive airways disease MESHD (COPD), and cancer are all associated with worse outcomes. The largest study, using a large nationwide sample of COVID-19 patients in China, found that those with multiple comorbidities had more than twice the risk of a severe outcome or death MESHD compared with patients with no comorbidities, after adjusting for age TRANS and smoking (HR=2.59, 95% CI 1.61, 4.17). Conclusions: This review summarises for clinicians, policymakers, and academics the most robust evidence to date on this topic, to inform the management of patients and control measures for tackling the pandemic. Given the intersection of comorbidity with ethnicity and social disadvantage, these findings also have important implications for health inequalities. As the pandemic develops, further research should confirm these trends in other settings outside China and explore mechanisms by which various underlying health conditions increase risk of severe COVID-19.