Thailand is attempting to implement an effective health insurance scheme to cover all migrant workers in the country. One of the remarkable policies is the Social Security Scheme (SSS). This study aims to assess the equity of SSS coverage among migrant workers in Thailand, sorted by types of businesses (agriculture, services and industrial sectors) and Gross Provincial Product (GPP) per capita. A secondary data analysis on time series cross-sectional data was employed. The dataset comprised: (1) the number of migrant insurees under the SSS; (2) the volume of migrant insurees in formal and informal sectors; and (3) provincial economic level and provincial population by years from 2015-2018. Descriptive statistics, Spearman's rank correlation, and concentration index analysis on the ratio of SSS insurees to all migrants ranked by GPP per capita and business types from 2015-2018 were performed. Results showed that the ratio of SSS insurees to all migrants increased from 38.5% to 58.9%. Spearman's correlation found a positive relationship between the SSS coverage and GPP per capita and business types in 2018. The statistical significance (p < 0.001) was found only in the industrial sector (rs = 0.346). Significant CIs were found in SSS coverage in the industrial sector in 2016 (CI = 0.147, p < 0.001), and SSS coverage in the industrial sector in 2017 (CI = 0.137, p < 0.001). In conclusion, the trend of CIs displayed a movement towards zero for all business types. This implied that the distribution of migrants in SSS shifted toward the equitable distribution across provinces in Thailand. A possible explanation was a major change in Thai politics in 2014 and a change in employment legislation for migrants in 2017. To expand the insurance coverage, the government should use the CI as a guide to consider specific provinces or target specific economic sectors as a priority to expedite the insurance enrolment for migrants.
Keywords: Thailand; concentration curve; concentration index; equity; income-related inequality; inequality decomposition; migrant workers; social security scheme.