Unraveling the role of viral interference in disrupting biennial RSV epidemics in northern Stockholm.

Respiratory syncytial virus (RSV) primarily impacts infants and older adults, with seasonal winter outbreaks in temperate countries. Delayed RSV activity was reported worldwide during the 2009 influenza pandemic, and a disrupted biennial pattern of RSV activity was observed in northern Stockholm following the pandemic. Biennial patterns shifted to early/large outbreaks in even-numbered years and late/small outbreaks in odd-numbered years. However, the mechanisms underpinning this change in pattern remain unknown. In this work, we construct an age-stratified mechanistic model to explicitly test three factors that can lead to the change in RSV transmission dynamics: (1) birth rates, (2) temperatures, and (3) viral interference. By fitting the model to weekly RSV admission data over a 20-year period and comparing different models, we find that viral interference from influenza is the only mechanism that explains the shifted biennial pattern. We further demonstrate that the pandemic H1N1 virus has the strongest viral interference effects with RSV, aligning with a previous in vivo study. Our work demonstrates the complex interplay between different respiratory viruses, providing evidence that supports the presence of interactions between the H1N1 pandemic influenza virus and RSV at the population level, with implications for future public health interventions.