Minimum time interval adjustment for 4-3-1 immunization rates among two-year-old children

Background: The purpose is to determine the administrative validity of the 4-3-1 immunization rates for DPT-OPV-MMR vaccines for 2-year-old children in a community health status assessment project by accounting for premature administration of specific vaccine doses according to ACIP recommended minimal timing intervals. Methods: A retrospective survey of immunization certificates was made on a random sample of 1,059 kindergarten enrollees in the county, including public, private, and parochial schools. The immunization rates by the crude 4-3-1 counting method were compared with the same method adjusted for minimal time interval vaccine dosing. Results: By the crude 4-3-1 counting method, 55.5% of the students had complete immunizations by their second birthdate, and 11.8% did not meet the minimum interval recommendations on at least one of 4-3-1 vaccine series. The adjustment for minimum time interval reduced the percent in compliance with the 4-3-1 counting method by age 2 in the community to 50.7%. Conclusion: The premature timing of vaccine doses is a threat to the validity of the 4-3-1 counting method. The crude 4-3-1 method over-estimates the completed immunization rates for 2-year-olds in this community-based study by about 4.8%.