Influenza epidemiology and immunization during pregnancy: final report of a World Health Organization working group by Fell, DB, Azziz-Baumgartner, E, Baker, MG, Batra, M, Beauté, J, Beutels, P, Bhat, N, Bhutta, ZA, Cohen, C, De Mucio, B, Gessner, BD, Gravett, MG, Katz, MA, Knight, M, Lee, VJ, Loeb, M, Luteijn, JM, Marshall, H, Nair, H, Pottie, K, Salam, RA, Savitz, DA, Serruya, SJ, Skidmore, B