National influenza surveillance systems in five European countries: a qualitative comparative framework based on WHO guidance door de Fougerolles, TR, Damm, O, Ansaldi, F, Chironna, M, Crépey, P, de Lusignan, S, Gray, I, Guillen, JM, Kassianos, G, Mosnier, A, de Lejarazu, RO, Pariani, E, Puig-Barbera, J, Schelling, J, Trippi, F, Vanhems, P, Wahle, K, Watkins, J, Rasuli, A, Vitoux, O, Bricout, H