Risk Factors for Delayed Immunization in a Random Sample of 1163 Children From Oregon and Washington

Despite extensive study of vaccine safety and decades of effort to immunize infants and toddlers, little is known about the comprehensiveness of vaccine coverage in US children younger than 2 years of age. Provider and parent data from a population-based sample of 1163 children from two states were analyzed to assess coverage rates at three ages and to evaluate characteristics of children and their families that predict failure to immunize on schedule. Overall, 78% of the children had received their first dose of diphtheria and tetanus toxoids with pertussis vaccine (DTP) and their first dose of oral poliovirus (OPV) by 92 days of age. Similarly, 77% had received their third dose of DTP and their second dose of OPV by their first birthday. However, by their second birthday only 60% had received the full series of four doses of DTP, three doses of OPV, and one dose of the measles, mumps, and rubella vaccines. When considered singly, several variables including child birth order, family income, maternal education, and marital status significantly predicted failure to immunize on schedule. In multivariate logistic models, only birth order and maternal education consistently predicted vaccine status at each of the three ages. Compared with first-born children, those who were later-born were 1.7 times more likely to be incompletely immunized at 2 years of age (95% confidence interval: 1.2, 2.3). Children of more educated mothers were significantly less likely to be underimmunized at all ages. These data highlight the general need to continue promoting immunizations to the broad population of US parents and a specific need to aggressively target children who are later-born or have unmarried, low-income, or poorly educated mothers.