Predicting regional and temporal incidence of RSV and influenza hospitalizations in a birth cohort of young Australian children.

Western Australia (WA) experiences multiple climatic zones, influencing the epidemiology of respiratory viruses. We aimed to estimate the true incidence of respiratory syncytial virus (RSV) and influenza hospitalizations across these different climatic regions using predictive modelling. We conducted a population-based cohort study using linked perinatal, hospitalization and microbiological data of children aged < 5 years, born in WA between 1 January 2010 and 31 December 2021. We used multivariable logistic regression to develop and validate predictive models for RSV and influenza hospitalizations in three climatic regions: southern temperate, northern tropical, and central desert. We compared laboratory-confirmed hospitalization rates with model-predicted rates and determined under-ascertainment fractions. Our cohort comprised 466,037 hospital admissions (257,960 children), with 33,106 tested for RSV (23.4% positive) and 33,511 for influenza (3.6% positive). True hospitalization rates for RSV and influenza were highest in the central desert and northern tropical regions. Among infants, RSV rates were 36.71 versus 20.00 per 1,000 child-years, and influenza rates were 444.59 versus 144.40 per 100,000 child-years in central desert compared to southern temperate areas. Routine laboratory testing significantly underestimated RSV (by 45-69%) and influenza (by 34-52%) hospitalizations. Unlike the southern and desert regions, the northern tropics lacked clear seasonal patterns. The study highlights high under-ascertainment of RSV and influenza hospitalizations from routine viral testing. The findings suggest immunization programs should consider viral circulation timing in different climatic regions. Prediction models demonstrated reliability in estimating RSV and influenza burden across varying climates, supporting localized decision-making beyond Australia.