1. Division of Allergy and Infectious Diseases, University of Washington, Seattle, Washington, USA
2. Department of Obstetrics, Gynecology, and Reproductive Sciences, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania, USA
3. University of Pittsburgh Magee-Womens Hospital, Pittsburgh, Pennsylvania, USA
4. Department of Obstetrics and Gynecology, Weill Cornell Medicine, New York Presbyterian Hospital, New York, New York, USA
5. Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia, USA