Nirsevimab Real‑world Effectiveness and Early Pathogen Shifts in Italian Infant LRTI Hospitalizations

A nationwide, retrospective multicenter analysis across 19 Italian pediatric centers describes infants hospitalized for lower respiratory tract infection (LRTI) during the first RSV season after nirsevimab became available. Within this hospitalized cohort, the authors report lower adjusted odds of RSV-related hospitalization among infants with prior immunization compared with those without documented prophylaxis. The report also summarizes detected respiratory viruses and the timing patterns described during that first post-rollout season.

The study retrospectively captured LRTI hospitalizations from November 2024 through April 2025 and analyzed 401 hospitalized infants younger than 24 months with prophylaxis status and respiratory testing documented. Using a multivariable logistic regression model adjusted for age, sex, and month of diagnosis, the authors report an association between prior nirsevimab immunization and reduced likelihood of RSV-related hospitalization (aOR 0.259; 95% CI 0.157–0.427). They translate this to an estimated immunization effectiveness of 74.1% (95% CI 57.3–84.3), framed as an adjusted association observed among hospitalized infants during the first season with nationwide availability.

Across all LRTI hospitalizations, the authors report RSV as the most frequently identified pathogen (47.5%), followed by rhinovirus/enterovirus (20.2%) and human metapneumovirus (hMPV; 6.9%). In pathogen-specific adjusted analyses, they also report higher odds of hMPV being identified among hospitalizations in nirsevimab-immunized infants (aOR 2.490; 95% CI 1.019–6.085) and higher odds of rhinovirus/enterovirus being identified among hospitalizations in immunized infants (aOR 2.573; 95% CI 1.424–4.650). Within the hospitalized cohort, prior nirsevimab immunization was associated with higher adjusted odds of hMPV and rhinovirus/enterovirus identification among LRTI admissions, without establishing a population-level change in viral epidemiology.

Timing patterns in the report place hMPV-associated LRTIs later than the typical RSV peak, with hMPV detections described as occurring outside the usual RSV-season window. Etiology assignment relied on RT-PCR respiratory virus panel testing of respiratory specimens, and the authors note that bacterial detections were rare. They also describe hMPV-associated LRTIs as being linked to moderate-to-severe clinical presentations in this dataset, aligning later-season presentations with a testing-based classification of viral causes during the 2024–2025 surveillance window.

Key Takeaways:

• In adjusted analyses of hospitalized infants during the first post-rollout season, the authors report lower odds of RSV-related hospitalization and provide an effectiveness estimate for prior immunization.
• The authors report RSV as the most commonly detected virus among LRTI hospitalizations, with rhinovirus/enterovirus and hMPV also identified, and they describe higher adjusted odds of hMPV and rhinovirus/enterovirus identification among hospitalizations in immunized infants.
• Later-season timing for hMPV relative to RSV was reported, with RT-PCR panel testing used for viral identification and bacterial detections described as rare.