1. Comprehensive Family Immunization Unit, Pan American Health Organization, Regional Office of the World Health Organization, Washington, DC, United States of America. Send correspondence to Lucia Helena De Oliveira, oliveirl@paho.org
2. Comprehensive Family Immunization Unit, Pan American Health Organization, Regional Office of the World Health Organization, Washington, DC, United States of America.
3. Albert B. Sabin Vaccine Institute, Washington, DC, United States.
4. Hospital Gutierrez, Buenos Aires, Argentina.