Infant Immunization Coverage in the SCHIP Era: An Instrumental Variables Approach

Background: Childhood immunization coverage is an important measure of health care utilization because of its efficacy in preventing contagious diseases. Declining vaccination rates contribute to outbreaks of preventable diseases. Previous studies suggest that access to health care is pivotal in increasing vaccination rates. The 1997 State Children’s Health Insurance Program (SCHIP) expands public health insurance coverage for children, and covers the cost and administration of all childhood immunizations recommended by the Advisory Committee on Immunization Practices.  Objective: To examine the impact of SCHIP/Medicaid income eligibility expansion on the probability of receiving recommended vaccines in a timely manner (i.e. without a delay of more than 6 months from the recommended age of vaccination) among children aged 19-23 months.  Methods: Using data from the 1996-2002 National Immunization Survey (NIS) and the Survey of Income and Program Participation (SIPP), a simulated instrumental variable (IV) strategy was implemented to assess the effects of SCHIP/Medicaid income eligibility expansions on children’s vaccination status in a timely manner. The idea is to use a measure of the generosity of state’s public insurance program towards children in a specific year from the SIPP data, the simulated eligibility fraction, to instrument for children’s actual insurance income eligibility based on gross income thresholds and family size. The “simulated instrument” only varies with a state’s legislative environment but not with its economic or demographic characteristics. By using a constant national sample to construct the IV, I purge the estimates of the state and year-specific economic effects which might be correlated with both income eligibility and vaccination service utilization. To address the concern that children who are always ineligible for SCHIP/Medicaid are sufficiently different from children who are sensitive to income eligibility cutoffs, a subgroup analysis on children with family income less than 400 percent of the Federal Poverty Level was also conducted. Reasonable bounds of the point estimate are also provided.  The outcome variables are NIS measures on the up-to-date vaccination rate for 4:3:1:3 vaccine series (four doses of diphtheria, tetanus and acellular pertussis vaccine, three doses of polio vaccine, one dose of measles containing vaccine, and three doses of Haemophilus influenza vaccine), Hepatitis B vaccine series, and varicella.  Results: The results suggest that being eligible for SCHIP/Medicaid coverage is associated with a higher probability of being up-to-date for the 4:3:1:3 vaccine series and Hepatitis B vaccine series without a lengthy delay by 10 and 13 percentage points, respectively. The effect on varicella vaccine is positive but not statistically significant. These results corroborate prior findings that access to health care may play an important role in increasing immunization rates. The improved vaccination coverage is most pronounced among children in states that had the highest rate of uninsured children before the SCHIP expansions.  Conclusion:  Given that timely receipt of vaccine maximizes the benefits associated with childhood vaccination, it is important to study associated factors. This study suggests that increased access to health care through SCHIP expansions is associated with improved preventive health care utilization of children with low socioeconomic status.