BackgroundIdentifying risk factors for respiratory syncytial virus (RSV)–associated severe acute respiratory illness (SARI) will assist with targeting vaccine interventions.MethodsUsing surveillance data from South Africa (2012–2018), we compared the characteristics of individuals with RSV-associated influenza-like illness (ILI) (reference group) to those with RSV-associated SARI to describe factors associated with SARI using a multivariable analysis.ResultsRSV was detected in 6% (483/7792) of ILI cases and 15% (844/5672) of SARI cases. Factors associated with SARI in children included age < 2 months, compared to age 2–4 years (adjusted odds ratio (aOR) 54.4; 95% confidence interval (CI) 23.5–125.8), malnutrition (aOR 1.9; 95% CI 1.2–3.2), prematurity (aOR 2.4; 95% CI 1.3–4.6) and living with HIV (LWH) (aOR 22.5; 95% CI 2.9–174.3). In individuals ≥ 5 years, factors associated with SARI included age ≥ 65 years compared to age 5–24 years (aOR 10.7; 95% CI 1.1–107.5), symptom duration ≥ 5 days (aOR 2.7; 95% CI 1.1–6.3), underlying illness (aOR 2.7; 95% CI 1.5–26.1) and LWH (aOR 16.8, 95% CI: 4.8–58.2).ConclusionIndividuals at the extremes of age and those with identified risk factors might benefit most from RSV prevention interventions.Clinical trial numberNot applicable, this is not a clinical trial.