1. El Salvador National Institute of Health, San Salvador 1101, El Salvador
2. Division of Pediatric Tropical Medicine, Department of Pediatrics, Baylor College of Medicine and Texas Children’s Hospital, Houston, TX 77030, USA
3. William T. Shearer Center for Human Immunobiology, Texas Children’s Hospital, Houston, TX 77030, USA
4. Centers for Disease Control and Prevention, Central America Office, Guatemala City 01015, Guatemala
5. Department of Pediatrics and Children’s Healthcare of Atlanta, Emory University, Atlanta, GA 30033, USA