Did the Medicaid expansion improve immunization among U.S. pregnant women?

Objectives: Affordable Care Act (ACA) Medicaid expansion increased financial eligibility of low-income individuals. Its effects are mixed on influenza vaccination and unknown for Tetanus, Diphtheria, and Acellular Pertussis (Tdap) vaccination among pregnant women in the United States. We aim to address mixed and missing evidence in literature. Methods: We merged 44,320 pregnant women from 2011 to 15 Pregnancy Risk Assessment Monitoring System with National Welfare Data by residency. Treatment group included Medicaid enrollees in post-ACA expansion states. Control included non-Medicaid enrollees in expansion states and all individuals in non-expansion states. We employed triple-difference method to estimate Medicaid expansion treatment effects on individual vaccination probabilities, and associations with socioeconomics and demographics. Results: Treatment group did not differ from control to vaccinate due to Medicaid expansion. Additionally, post-ACA Medicaid enrollees were 4.5-9 % less likely to vaccinate than pre-ACA non-Medicaid enrollees. Medicaid enrollees in expansion states were 2.5 % less likely to vaccinate against influenza than non-Medicaid enrollees in non-expansion states. African Americans were 4-5 % less likely, while Native Americans were 3-7 % more likely, to vaccinate than counterparts. Those with low parity and college education were 3.2-5.4 % and 4.8-10.6 % more likely to vaccinate. Individuals in high-poverty states were 0.6 % less likely to receive the influenza vaccine. Conclusions: We contributed different national data on understudied populations and the triple-difference method to the literature. Income-based Medicaid expansion did not increase the likelihood of vaccination among pregnant women. Non-income-based policies may target Medicaid-enrolled pregnant women in expansion states for influenza vaccination. Future interventions may target high-parity, low-education, and African American pregnant women in high-poverty non-expansion states.