Effectiveness depends on adherence

databases to estimate the proportion of vaccinated infants who complete the dosing schedule, but excluded from analysis vaccinated infants who were not enrolled in the database for a sufficient period to observe all 3 doses. Restricting study populations based on duration of follow-up can introduce bias if a large number of subjects are excluded due to insufficient follow-up, and if their outcomes differ from subjects who are included. To address the possibility that exclusions may have been extensive and led to biased