The Neutrophil/Lymphocyte Ratio Was Identified as a Marker of Severe Influenza During the 2024–2025 Outbreak in France

TL;DR

A high neutrophil/lymphocyte ratio was found to predict severe influenza cases during the 2024–2025 outbreak in France.

Contribution

The study identifies the neutrophil/lymphocyte ratio as a novel, easily measurable predictor of severe influenza.

Findings

A neutrophil/lymphocyte ratio > 15 was independently associated with severe influenza (ORadj 8.79).

The ratio had 94.6% specificity and 81.8% positive predictive value for predicting severe cases.

Severe cases showed higher neutrophil/lymphocyte and platelet/lymphocyte ratios upon admission.

Abstract

Background/Objectives: Influenza continues to cause high morbidity and mortality rates worldwide, inflicting a major burden on the public health system. There is little data available on the 2024–2025 seasonal outbreak. Moreover, biomarkers for rapidly identifying subjects at higher risk for severe forms are needed. Methods: We retrospectively collected hospitalization data for influenza in Cannes, France, during the 2024–2025 seasonal outbreak. Severe forms were defined as cases either requiring admission to the Intensive Care Unit (ICU) or resulting in death. They were compared to uncomplicated forms. Main demographic, clinical, radiological, and laboratory characteristics were collected for each patient. Results: From October 2024 to May 2025, 59 patients were admitted to either the Infectious Diseases Department or the ICU (56% male, age 72 years, 27% vaccinated, influenza type A 93%, symptom duration 3.5 days prior to hospitalization, 31% admissions to ICU, 14% deaths). Vaccination status did not differ between severe and uncomplicated forms. In the univariate analysis, severe forms had higher neutrophil/lymphocyte and platelet/lymphocyte ratios upon admission and included more cases of acute hepatitis, pneumonia, and oseltamivir use than uncomplicated forms. A neutrophil/lymphocyte ratio > 15 was independently associated with severity (ORadj 8.79, 95% CI: 1.34–57.6, p = 0.023), with 40.9% sensitivity, 94.6% specificity, 81.8% positive predictive value, and 72.3% negative predictive value for predicting a severe form. Conclusions: The N/L ratio was an easy-to-perform predictive marker for influenza severity during the 2024–2025 seasonal outbreak, warranting further prospective studies