1. Infectious Disease Section, Department of Pediatrics, Baylor College of Medicine, Houston, Texas
2. Department of Obstetrics and Gynecology, Baylor College of Medicine, Houston, Texas
3. Department of Statistics, Baylor College of Medicine, Houston, Texas
4. Pertussis and Diphtheria Laboratory, Centers for Disease Control and Prevention, Atlanta, Georgia
5. Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas
6. Now with Department of Pediatrics, The University of Texas Health Science Center at Houston