Decreased effectiveness of the influenza A(H1N1)pdm09 strain in live attenuated influenza vaccines: an observational bias or a technical challenge? Penttinen, Pasi M and Friede, Martin H, Eurosurveillance, 21, 30350 (2016), https://doi.org/10.2807/1560-7917.ES.2016.21.38.30350