Abstract
BACKGROUND: Parainfluenza viruses are significant contributors to childhood respiratory illness worldwide, although detailed epidemiological studies are lacking. Few recent Australian studies have investigated serotype-specific PIV epidemiology, and there is a paucity of southern hemisphere PIV reports. We report age-stratified PIV hospitalisation rates and a mathematical model of PIV seasonality and dynamics in Western Australia (WA). METHODS: We used linked perinatal, hospital admission and laboratory diagnostic data of 469 589 children born in WA between 1996 and 2012. Age-specific rates of viral testing and PIV detection in hospitalised children were determined using person time-at-risk analysis. PIV seasonality was modelled using a compartmental SEIRS model and complex demodulation methods. RESULTS: From 2000 to 2012, 9% (n = 43 627) of hospitalised children underwent PIV testing, of which 5% (n = 2218) were positive for PIV-1, 2 or 3. The highest incidence was in children aged 1-5 months (PIV-1:62.6 per 100 000 child-years, PIV-2:26.3/100 000, PIV-3:256/100 000), and hospitalisation rates were three times higher for Aboriginal children compared with non-Aboriginal children overall (IRR: 2.93). PIV-1 peaked in the autumn of even-numbered years, and PIV-3 annually in the spring, whereas PIV-2 had inconsistent peak timing. Fitting models to the higher incidence serotypes estimated reproduction numbers of 1.24 (PIV-1) and 1.72 (PIV-3). CONCLUSION: PIV-1 and 3 are significant contributors towards infant respiratory hospitalisations. Interventions should prioritise children in the first 6 months of life, with respect to the observed autumn PIV-1 and spring PIV-3 activity peaks. Continued surveillance of all serotypes and investigation into PIV-1 and 3 interventions should be prioritised.