Climate drivers and seasonality of respiratory viruses in the Southern Hemisphere: Similarities and differences between Respiratory Syncytial Virus and Influenza A

Background/Question/Methods

While environmental factors have been shown to partially explain the seasonality of infectious diseases, a better understanding of the link between climatic factors and the transmission of respiratory diseases is needed, especially in the Southern Hemisphere, with potential implications for our ability to predict the timing and magnitude of outbreaks. Recently available data from Chile spanning 9 years of weekly lab-confirmed cases provide a unique opportunity to understand the effect of temperature and humidity on seasonality and inter-annual variation of respiratory viruses like Respiratory Syncytial Virus (RSV) and Influenza A. The climate of Chile comprises a wide range of weather conditions across a large geographic scale, and its long latitudinal gradient makes it a suitable place to study how environmental drivers affect the transmission of infectious diseases. We examine the association between climatic factors and positive confirmed cases by looking at the timing of RSV and Influenza A onset and inter-annual variability by implementing statistical and mechanistic epidemiological models.

Results/Conclusions

While there is no clear signal of climate factors influencing inter-annual variability for both RSV and Influenza A, we do observe a consistent seasonal forcing that can be explained by environmental factors, which is stronger for RSV. Absolute humidity and temperature are significantly associated with the mean timing onset of RSV (p< 0.01), and explain 54% and 51% of the variance, respectively. In the case of Influenza A, this association is weaker, where absolute humidity (p = 0.09) and temperature (p = 0.02) only explain 19% and 34% of the variance, respectively. Differences in the age of infection and contact patterns across different age groups could explain some of the differences between RSV and Influenza A. RSV infects mainly children while influenza A infects a wide range of ages, mostly adults. Immune history and connectivity across different regions might also interfere with the role of climate factors in the transmission of Influenza A. Outcomes from mechanistic models that take into account intrinsic and extrinsic factors can be useful to further understand these results in the context of seasonality, inter-annual variability, and climate change.