Factors associated with recruitment, surveillance participation, and retention in an observational study of pregnant women and influenza oleh Mark G. Thompson, De-Kun Li, Allison L. Naleway, Jeannette R. Ferber, Michelle L. Henninger, Pat Shifflett, Leslie Z. Sokolow, Roxana Odouli, Tia L. Kauffman, Rebecca V. Fink, Joanna Bulkley, Janet D. Cragan, Sam Bozeman