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.