The deceased children ranged in age from newborn to 18 years, and represented the full range of medical and surgical diagnoses.

Immunization coverage and timing varied widely by country, as did the proportion of children immunized and with what vaccines. The mean age at first immunization was 1.1 months.

Because cerebral metabolism appears to change continuously during childhood, beneficial or harmful effects that were limited to a very specific age range may have been diluted in the larger analysis.