BACKGROUND: In 2014, there were 2,576 pertussis cases and 2 pertussis deaths in Texas. Immunizations are one of the most effective public health tools for reducing the morbidity and mortality of vaccine-preventable diseases such as pertussis. To ensure children are protected against vaccine-preventable diseases vaccines must be administered age appropriately according to the recommended immunization schedule. Currently, one way Texas assesses immunization coverage among children at the state and local level is through the National Immunization Survey (NIS). The NIS provides immunization coverage levels by determining the proportion of children who are up to date with recommended vaccines. NIS coverage does not provide information on whether immunizations were received age appropriately. Children with delayed immunizations are subject to periods of vulnerability to vaccine-preventable diseases and can contribute to the spread of disease. This study aims to estimate the proportion of Texas children vaccinated on time for the fourth dose of DTaP vaccine and identify sociodemographic differences in age appropriate vaccination.
METHODS: NIS data files were used to determine ages and dates of vaccination of surveyed children in Texas. Unvaccinated children and invalid doses were not considered in this analysis because the focus was to assess delay. Survival analysis, also known as the Kaplan-Meier method, was used to assess deviations over time from the recommended DTaP vaccine schedule. Survival analysis allows calculation of the cumulative probability of the population vaccinated by any given age in days or months. Inverse Kaplan-Meier curves were produced to visualize the increase in proportion of children vaccinated over time overall, and observe trends in vaccination by education, race, and insurance status.
RESULTS: By age 19 months, 82 percent of Texas children vaccinated with the fourth dose of DTaP received it on time. By age 20 months, that number rose to 88 percent, and by 2 years of age, 95 percent had received their fourth DTaP. The median delay in DTaP vaccination was 47 days. Differences in age appropriate fourth DTaP vaccination were observed among different sociodemographic groups.
CONCLUSIONS: Survival analysis is an efficient method for calculating and visualizing the proportion of children vaccinated age appropriately. Over 80 percent of children who received their fourth dose of DTaP received it on time. Assessment of delay in age appropriate vaccination in combination with up-to-date coverage allows for a more comprehensive analysis of immunization coverage and can help monitor trends in delay of vaccination that make children vulnerable to vaccine-preventable disease.