Respiratory Syncytial Virus Epidemiology in Argentina From COVID-19 Pandemic to the Maternal Immunization Strategy

Introduction: Worldwide, respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory tract infections (LRTIs) and hospital admissions among infants and young children. The COVID-19 pandemic changed the epidemiology and clinical patterns of respiratory viruses other than severe acute respiratory syndrome coronavirus 2. Argentina introduced the RSV maternal vaccine in 2024. This multicenter study describes the clinical-epidemiological profile of hospitalized pediatric patients with LRTI associated with RSV in Argentina, comparing pre- and postpandemic periods, and identifies independent predictors of RSV infection. Materials and methods: This prospective, multicenter study included patients under 18 years old admitted for LRTI in 5 tertiary centers in Argentina before (2018-2019) and after (2022-2023) COVID-19. Changes in viral detection rates, seasonality, epidemiological and clinical characteristics were analyzed. Indirect immunoassay or real-time polymerase chain reaction were used for virological diagnosis prepandemic and real-time polymerase chain reaction for postpandemic. Data analysis was performed using Epi Info 7. Results: A total of 5838 LRTI cases were included (mean age: 9.5 months; interquartile range: 4-22 months), with 96.6% tested for viral detection, and 66.4% positive (3877 cases). RSV was the most prevalent virus, followed by parainfluenza and influenza. Postpandemic, there was a significant decrease in RSV prevalence and an increase in parainfluenza and metapneumovirus infections. LRTI seasonality shifted 6 weeks forward postpandemic. In 2022, metapneumovirus cases increased, displacing RSV, which recovered prevalence and typical seasonality in 2023 with an earlier onset. Viral coinfection occurred in 17.2% of RSV cases. Globally, 71% of RSV cases were infants under 12 months (45.8%, <6 months); bronchiolitis was the most common clinical presentation (68.9%), with cough (78.3%), respiratory distress (69.6%), rhinorrhea (65.9%) and fever (64.7%) being the most common symptoms. Nearly 44% had underlying conditions, 15.7% were born preterm, 15% required intensive care and 24% received empirical antibiotics. The pre- and postpandemic periods showed no age differences in RSV-associated LRTI cases, but higher comorbidity prevalence, intensive care requirement and lower empirical antibiotic use postpandemic. Prematurity [odds ratio (OR): 1.3, 95% confidence interval (CI): 1.1-1.5; P = 0.004], comorbidities (OR: 1.8, 95% CI: 1.6-2; P < 0.001) and age under 6 months (OR: 1.8, 95% CI: 1.6-2.1; P < 0.001) were independent predictors of RSV infection. Conclusions: RSV primarily affected infants under 1 year old. No age differences were found between LRTI cases associated with RSV pre- and postpandemic. Postpandemic, there was a higher prevalence of comorbidities, increased intensive care requirements and lower empirical antibiotic use. Prematurity, underlying conditions and age under 6 months were independent predictors of RSV infection in hospitalized LRTI patients.