COVID-19 disruption and rebound of respiratory syncytial virus hospital burden in Australia: A national interrupted time-series analysis.

BACKGROUND: COVID-19 non-pharmaceutical interventions disrupted respiratory syncytial virus (RSV) transmission globally. In Australia, RSV surged back after restrictions eased, whether this reflected immunity debt repayment or epidemic amplification is untested. METHODS: We conducted a national interrupted time-series analysis using Australian Institute of Health and Welfare hospitalisation data from financial years (FY) 2010/11-2023/24. The primary outcome was RSV-attributable fraction (RSV-AF) of bronchiolitis in infants and children 1-4 years. Pre-pandemic counterfactual trends were estimated by linear extrapolation of FY2010/11-2019/20 training data, this was appropriate given the stable, low-gradient pre-pandemic RSV-AF trends in both groups. For adults ≥ 65 years, a flat counterfactual (pre-pandemic mean) to bound estimates where a secular ascertainment rise made linear extrapolation inappropriate. Formal amplification test: one-sided Z-test of whether rebound exceeded suppression deficit. Influenza-attributable fraction used as a pathogen-specific comparator to isolate RSV from general respiratory rebound. Non-RSV bronchiolitis decomposed to rule out general post-pandemic denominator effects. RESULTS: In infants, pre-pandemic RSV-AF was 32.3% (SD 1.8%). In FY 2022/23, it reached 52.0% (z = 10.78; p < 0.001), 19.7 %age points above the pre-pandemic upper control limit. RSV bronchiolitis accounted for 95.1% of the total bronchiolitis increase; non-RSV bronchiolitis remained below counterfactual. In 1-4-year-olds, RSV-AF of bronchiolitis reached 49.1%, against a pre-pandemic mean of 23.2%. Formal testing found rebound significantly exceeded deficit in infants, 1-4-year-olds, and 5-19-year-olds (all p < 0.001). Adults 20-64 years showed significantly incomplete discharge (β=0.71, meaning the rebound recovered only 71% of the accumulated suppression deficit; p = 0.020). CONCLUSIONS: The RSV rebound in young children significantly exceeded the suppression deficit, consistent with epidemic amplification potentially reflecting accumulated primary-infection susceptibility in unvaccinated cohorts. After explanations (testing intensity, admission thresholds, demography, and coding changes) were accounted for. The FY2022/23 baseline provides the pre-vaccination reference for evaluating Australia's RSV immunisation program.