The incidence of infections from arthropod-borne viruses, including chikungunya and dengue, is increasing globally. We used nationwide data collected over a decade in Brazil to examine the factors associated with hospitalisation, in-hospital mortality, and the years of life lost from these diseases in Brazil. Using nationwide de-identified chikungunya and dengue disease records registered from 1st January 2015 to 31 December 2024, we estimated the risk factors for hospitalisation and in-hospital mortality via logistic regression and the Fine and Gray model, respectively. We also calculated the years of life lost for each disease and the average of years of life lost (aYLL), stratified by geographic region, sex and race/ethnicity. We studied 1,125,209 chikungunya cases: 21,336 (1.9%) required hospitalisations. Among hospitalised cases, 1044 (4.9%) deaths occurred within 84 days of symptom onset, of which 728 (69.7%) were attributed to chikungunya. We studied 13,741,408 dengue cases: 455,899 (3.3%) required hospitalisation, with 12,969 (2.8%) deaths among the hospitalised cases, with 9989 (77.0%) attributed to dengue. Age (<1 or ≥70 years), sex (male), and the presence of diabetes and kidney disease were risk factors for hospitalisation and in-hospital mortality in both diseases. The aYLL for chikungunya was 16.0 years, and for dengue, 14.5 years; however, the burden was not evenly distributed across the population. For chikungunya, Black participants experienced the highest aYLL of 22.0 years, while White participants were the least affected (aYLL: 13.0). For dengue, the most affected group was Indigenous (aYLL: 22.5) and the least White (aYLL: 12.6). Infants, older people (≥70 years), male sex and the presence of comorbidities are associated with increased severity in cases of chikungunya and dengue. These diseases disproportionately affect historically minoritised populations, with participants who self-identified as Black and Indigenous experiencing significantly greater years of life lost compared to the white population. Mitigating the impacts of chikungunya and dengue necessitates addressing health and social inequities. Royal Society, Wellcome Trust, CNPq. [Abstract copyright: © 2025 The Author(s).]
