# Circulation of RSV Subtypes A and B Among Mexican Children During the 2021–2022 and 2022–2023 Seasons

## TL;DR

This study tracks RSV subtypes A and B in Mexican children with pneumonia, revealing seasonal shifts and co-infections with other pathogens.

## Contribution

The study provides new insights into RSV subtype dynamics and co-infection patterns in Mexico during two consecutive seasons.

## Key findings

- RSV-B dominated in 2021–2022, while RSV-A was more prevalent in 2022–2023.
- 77% of RSV-positive samples showed co-infection with other respiratory pathogens like rhinovirus and Streptococcus pneumoniae.
- RSV-A lineages showed higher genetic diversity compared to RSV-B, with specific lineages frequently detected.

## Abstract

Respiratory syncytial virus (RSV) remains a leading cause of pneumonia in young children in Mexico and worldwide. To investigate RSV dynamics in Mexico, we conducted a multicenter study from August 2021 to July 2023 in six hospitals across five States, analyzing respiratory samples from children under five years with pneumonia. Multiplex RT-PCR identified 203 RSV-positive cases, of which 123 were RSV-B and 80 RSV-A. Interestingly, 77% of the collected samples showed evidence of coinfection with other respiratory pathogens, with rhinovirus, Haemophilus influenzae, and Streptococcus pneumoniae being the most common. Also, RSV-B dominated in 2021–2022, whereas RSV-A prevailed in 2022–2023, mirroring trends observed in the United States. Sequences of the genes encoding G and F proteins showed that RSV-A lineages were more diverse, with A.D.1, A.D.1.8, and A.D.5.2 being frequently detected. In contrast, nearly all RSV-B sequences belonged to lineage B.D.E.1. Finally, ancestral state inference suggests repeated introductions from the USA and other North American countries, with limited evidence of sustained local circulation. These findings show different trends in RSV circulation between two consecutive seasons and the importance of genomic surveillance to monitor RSV diversity, evaluate vaccine impact, and inform public health strategies in Mexico’s evolving post-pandemic respiratory virus landscape.