Estimating Sickle Cell Disease Prevalence By State: A Model Using US-Born and Foreign-Born State-Specific Population Data

Introduction: Epidemiological data on sickle cell disease (SCD) in the United States is incomplete, particularly at the state level. Current national prevalence estimates do not include SCD cases among people who migrate to the US from high prevalence areas and may result in an underestimation of the size of the prevalent population. To account for this gap, we constructed an epidemiological model that extrapolates SCD prevalent cases based on both US- and foreign-born populations in each state for 2021.

Methods: The total US SCD prevalent cases were estimated by the sum of US- and foreign-born SCD cases for each state. The US-born SCD cases in each state were calculated by applying the prevalence rates derived from new-born-screening studies to the US-born population. The foreign-born SCD cases were calculated by applying the average SCD prevalence rate from the literature for the Caribbean, Western Africa, and Middle and Southern Africa to the corresponding foreign-born populations of each state. These regions were selected due to their high endemic rates of SCD and availability of data required for the extrapolation. Other countries with high prevalence of SCD, such as other African regions, Middle Eastern countries, India, and Brazil, were not included in the analysis due to data limitations. Considering that people with SCD may have a lower propensity to migrate than the general population from these regions, we conservatively assumed that 25% of the total expected population of foreign-born SCD patients in each state would migrate to the US. The migration likelihood adjustment of 25% was based on benchmarking against state-specific SCD prevalence data published by the CDC and other modeling studies. A scenario with no migration adjustment was also explored.

Results: The model estimates that the total number of SCD cases in 50 states and Washington D.C. in the US was 120,156 cases in 2020, with 87% of the cases from the US-born population and 13% of the cases from immigrants from the Caribbean, Western Africa, and Middle and Southern Africa. Florida (13,886 cases), New York (11,715 cases), Texas (9,416 cases), Georgia (7,088 cases), and Maryland (5,812 cases) have the highest SCD burden. In nine states, prevalent SCD cases increased by more than 25% when the non-US-born SCD population is included. The proportion of total SCD cases attributable to immigrants varies widely by state (0-73%). When not adjusting for migration likelihood, the estimate of SCD prevalent cases in the US was 167,484 cases.

Conclusion: The prevalence of SCD and other hemoglobinopathies is growing in the US due to changing migration patterns. The total number of SCD prevalent cases estimated by the model is higher than the frequently cited national estimate of ~100,000 cases. We account for the difference by considering SCD cases from the immigrant population, which was lacking in previous national-level epidemiology estimates that use new-born screening methods. Implications of undercounting SCD cases is significant when considering healthcare resource planning. While the CDC's Sickle Cell Data Collection (SCDC) program is undertaking a significant effort to better understand SCD prevalence at the state level, the scope of the program does not include all US states. More research is needed to obtain accurate and up-to-date SCD epidemiology in the US.

Limitations: This extrapolation is limited by the accuracy of the US-born and foreign-born populations census reporting. US-born SCD prevalent cases may be underestimated as the new-born screening-based SCD prevalence used in the analysis may be outdated. The extrapolation only included the foreign-born population from three regions, which may underestimate SCD prevalent cases. This model does not account for inter-state migration patterns.