Effectiveness of nirsevimab against RSV hospitalisation in infants, Basque Country 2024-2025

Background

Respiratory syncytial virus (RSV) is the leading cause of lower respiratory tract infections and hospitalisations in infants. Universal immunisation with the long-acting monoclonal antibody nirsevimab has recently been implemented in several European regions, but real-world effectiveness and economic evaluations remain limited.

Methods

A retrospective population-based cohort study included all infants born in the Basque Country between July 1, 2024 and March 31, 2025. Data on births, immunisations and hospitalisations were linked using regional health information systems. RSV hospitalisations were defined as admissions for acute respiratory infection with laboratory-confirmed RSV. Incidence rates were calculated and a Cox proportional hazards model with time-dependent covariates estimated the effect of immunisation adjusting for age. A health system perspective cost analysis estimated the cost per hospitalisation avoided.

Results

The study included 9,635 infants, of whom 8,517 (88.4%) received nirsevimab. A total of 43 RSV hospitalisations were identified. Cumulative incidence of hospitalisation was 1.52% among non-immunised infants and 0.31% among immunised infants (p < 0.001). Incidence rates were 7.9 and 2.6 cases per 100,000 person-days respectively. In the Cox model, nirsevimab immunisation was associated with a 71% reduction in the hazard of RSV hospitalisation (HR 0.29; 95%CI 0.14–0.60; p = 0.001). Each additional day of age was associated with a 3% reduction in hazard (HR 0.97; 95%CI 0.96–0.98). Economic analysis estimated a net cost of €4,322 per hospitalisation avoided.

Conclusions

Universal nirsevimab immunisation in infants in the Basque Country was associated with a substantial reduction in RSV-related hospitalisations during the 2024–2025 season. These real-world results support the effectiveness of the programme and suggest the reduction in hospital burden was achieved with relatively low investment per hospitalisation prevented.

Key messages

• Universal nirsevimab immunisation reduced RSV hospitalisation risk by 71% in infants in the Basque Country during the 2024–2025 season in a real-world population-based study.

• The programme substantially reduced hospital burden, with an estimated net cost of €4,322 per hospitalisation avoided from the health system perspective.