Epidemiological and Clinical Changes in RSV-Associated Pneumonia in Children in Mexico Before and During the COVID 19 Pandemic

<b>Background/Objectives:</b> Respiratory syncytial virus (RSV) significantly affects young children. In 2020, at the beginning of the COVID-19 pandemic, widespread public health measures temporarily interrupted RSV transmission. However, by mid-2021, an atypical resurgence of RSV was observed. The objective of this study was to compare the clinical and epidemiological characteristics of RSV infections in children before and during the second half of the SARS-CoV-2 pandemic in Mexico. <b>Methods</b>: A comparative ambispective longitudinal epidemiological study was conducted using two distinct cohorts: one from 2010 to 2013 and another from 2021 to 2023. The study included children under five years of age diagnosed with RSV-related pneumonia. Statistical analyses included Student's <i>t</i>-tests, chi-square tests, and logistic regression to identify risk factors associated with severe pneumonia. Incidence density was calculated as the number of RSV-positive pneumonia cases per 10 new pneumonia admissions per month. <b>Results</b>: The mean age of affected children increased from 10 to 15 months. RSV activity began earlier in 2021, emerging during the summer months, and showed a higher incidence than in previous seasons. RSV type B was significantly more common during the pandemic period (58.5% vs. 3.8%), and the proportion of co-infections also increased (60% vs. 39%), indicating a change in the viral landscape. <b>Conclusions</b>: These findings indicate a shift in RSV seasonality toward summer and autumn, increased case incidence, and infections in older children. These observations underscore the need for ongoing surveillance to better understand evolving RSV patterns, especially in the context of complex public health scenarios like the COVID-19 pandemic.