Background: Live attenuated influenza vaccine(LAIV) use inchildren was suspended in the USA due to an apparentlack of protection against (H1N1)pdm09 virusesin the 2015/16 influenza season. The (H1N1)pdm09 vaccine strain was subsequently changed in 2017/18 to improve viral fitness. We conducted LAIV virus shedding studies to assess the effect of this change. Methods: Children aged2-18 yearswere recruited to receive quadrivalent LAIV in the 2016/17 (n=641) and 2017/18(n=362)influenza seasons. Viruses from nasal swabs taken 1, 3 and 6 days post-vaccination were quantified byreverse transcription-PCR and area under curve titres determined. Presence and quantity of shedding was compared between strains and seasons with adjustment for age and prior receipt of LAIV (n=436), inactivated seasonal (n=100) or (H1N1)pdm09 vaccine (n=166). Results: (H1N1)pdm09 detection (positivity) in 2016/17 and 2017/18 (11.2% and 3.9% respectively) was lower than that of H3N2 (19.7% and 18.7% respectively) and influenza B (28.9% and 33.9% respectively for B/Victoria). (H1N1)pdm09 positivity was significantly higher in 2016/17 than 2017/18 (p=0.005), butwithin participants that shed, the (H1N1)pdm09 titre increased in 2017/18 (p=0.02), whereasH3N2 and influenza B titres were similar between seasons. Positivity ratesdeclined with age and prior vaccination reduced the likelihood of shedding influenza B but not (H1N1)pdm09. Conclusions: The (H1N1)pdm09 titre increased in 2017/18, indicating more efficientvirusreplication in childrenthat shed virusthan the 2016/17 strain, although overall positivity was reduced. Age and prior vaccine receipt negatively correlated with shedding rates, therefore these factors require consideration when correlating virus shedding with clinical protection.