BACKGROUND: Respiratory Syncytial Virus (RSV) is a common cause of hospitalisation in infants worldwide, causing significant morbidity and mortality. Recently, the antiviral treatment, Ziresovir, has shown promising results in a Phase III trial conducted on infants hospitalised with RSV. Based on these topline results, this study aims to investigate the cost-effectiveness of Ziresovir in the United Kingdom (UK). METHODS: The cost-effective analysis (CEA) uses a proportional outcomes model using data from topline reports by the AIRFLO trial and published data to explore the effect of Ziresovir administration on hospitalised infants aged <24 months at admission. We estimated the reduction in ICU bed days and deaths and the maximum cost-effective price (MCEP) per treated individual, assuming a threshold of £20,000 per Quality-adjusted Life Year (QALY) gained. RESULTS: Administering Ziresovir to all hospitalised infants averts four deaths (range: 2.8-4.5), 216 ICU admissions (range:160-260) and 3169 ICU bed-days (range: 2348-3804) per annum, the MCEP for Ziresovir per hospitalised infant is £429.65 (95 % CrI: £236-£771). If preterm infants are targeted, then the MCEP increases to £2108.38 (95 % Crl: £870-£3540). The MCEP for exclusively treating Infants with Chronic Lung Disease (CLD) and Congenital Heart Disease (CHD) is £6557.24 (95 % Crl: £1250 - £14,920) and £9459.44 (95 % Crl £3350-£20,300) respectively. The model is highly sensitive to changes in the efficacy of Ziresovir and the risk of ICU admission and mortality. CONCLUSION: Ziresovir is a cost-effective intervention for all infants hospitalised with RSV if priced below £430 per dose and strategies that exclusively treat high-risk- with CLD and CHD infants justify a higher price of £6558 and £9460 respectively. The outcomes are highly sensitive to the efficacy of Ziresovir and can be improved when the full results of the AIRFLO trial are available.