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Background: Gut microbiota-derived short-chain fatty-acid (SFCA) acetate protects mice againstRSV A2 strain infection by increasing interferon-β production and expression of interferonstimulated genes (ISGs). However, the role of SFCAs in RSV infection using strains isolated from patients is unknown. Methods: We first used RSV clinical strains isolated from infants hospitalized with RSV bronchiolitis to investigate the effects of in vitro acetate treatment in human pulmonary epithelial cells. We next examined whether acetate treatment is beneficial in a mouse model of RSV infection using clinical isolates. We sought to investigate the relationship of gut microbiota and fecal acetate with disease severity among infants hospitalized with RSV bronchiolitis, and whether treating their respiratory epithelial cells with acetate ex-vivo impacts upon viral load and ISG expression. We further treated epithelial cells from SARS-CoV-2 infected patients with acetate. Findings: In vitro pre-treatment of A549 cells with acetate reduced RSV load after infection with clinical isolates and increased the expression of RIG-I and ISG15. Animals treated with acetate intranasally recovered significantly faster, with reduction in the RSV clinical isolates viral load, and increased lung expression of IFNB1 and the RIG-I receptor. Experiments in RIG-I knockout A549 cells demonstrated that the protection relies on RIG-I presence. Gut microbial profile was associated with bronchiolitis severity and with acetate in stool. Increased acetate levels were associated with increasing oxygen saturation at admission, and shorter duration of fever. Ex-vivo treatment of patients’ respiratory cells with acetate reduced RSV load and increased expression of ISGs OAS1 and ISG15, and virus recognition receptors MAVS and RIG-I, but not IFNB1. These acetate effects were not found on cells from SARS-CoV-2 infected patients. Interpretation: Acetate reduces the severity of RSV infection and RSV viral load through modulation of RIG-I expression. Funding: This study was supported by Rio Grande do Sul Research Foundation FAPERGS (FAPERGS/MS/CNPq/SESRS no. 03/2017 - PPSUS 17/2551-0001380-8 and COVID-19 20/2551-0000258-6), CNPq 312504/2017-9 and by Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior - Brasil (CAPES) - Finance Code 001. P.B., C.V.B. and L.A.B. would like to acknowledge financial support given by CNPq/FAPERGS/CAPES/BNDES to the National Institute of Science and Technology on Tuberculosis (INCT-TB), Brazil [grant numbers: 421703- 2017-2/17-1265-8/14.2.0914.1). Declaration of Interest: The authors declare no competing interests. Ethical Approval: All animal procedures were performed in accordance with protocols approved by CEUA/UNICAMP (protocols 4022-1 and 4599-1).