Haemophilus influenzae type b vaccine was licensed in the United States in the spring of 1985. Prior to licensure, extensive data had accumulated, indicating that this vaccine was safe and was immunogenic in children >18 to 23 months of age. Furthermore, in a large efficacy trial conducted in Finland, the vaccine also was found to be approximately 80% protective in preventing invasive type b Haemophilus disease in children 24 to 35 months of age (95% confidence interval 8, 95).1,2 Unfortunately, it was ineffective in children <18 months of age, the group most susceptible to Haemophilus disease, and it was of uncertain efficacy in those 18 to 23 months of age.

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