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Introduction. MicroRNA (miR) 155 has been implicated in the regulation of innate and adaptive immunity as well as antiviral responses, but its role during respiratory syncytial virus (RSV) infections is not known. The objective of this study was to investigate the expression of miR-155 using pharyngeal swabs and peripheral blood in infants with RSV infection and uninfected controls. Methods. A prospective age-matched study was conducted in primary care in Torino from 1 August 2018 to 31 January 2020. We enrolled 66 subjects, 29 of them patients with RSV infection and 37 age-matched uninfected controls, and collected pharyngeal swabs and peripheral blood in order to assess miR-155 expression with real-time stem–loop–TaqMan real-time PCR. Results. The data show that there is no correlation between pharyngeal swabs and peripheral blood with respect to miR-155 expression. The 1/ΔCq miR-155 expression levels in throat swabs in RSV bronchiolitis patients and healthy controls were 0.19 ± 0.11 and 0.21 ± 0.09, respectively, and were not significantly different between healthy controls and bronchiolitis (p = 0.8414). In the peripheral blood, miR-155 levels were higher than those of healthy control subjects: 0.1 ± 0.013 and 0.09 ± 0.0007, respectively; p = 0.0002. Discussion. Our data provide evidence that miR-155 expression is higher in peripheral blood during RSV infection but not in swabs. This difference in the timing of sample recruitment could explain the differences obtained in the results; miR-155 activation is probably only assessable in the very early stages of infection in the swab and remains visible for longer in the blood. New investigations are needed in order to clarify whether the miR-155 expression in swabs can be influenced by different stages of virus disease of infants. [ABSTRACT FROM AUTHOR]