This study found that socioeconomic inequality in untreated dental caries and sealant treatment was alleviated for children by an expansion of NHIS coverage in Korea.
After the coverage expansion of dental sealants, the prevalence of untreated dental caries decreased and that of having sealant treatment increased in both children and adolescents. This study also showed an overall increase in dental service usage after the coverage expansion . A similar review on smoking inequality in youth after tobacco control policies concluded that  education and information communication led to widening inequalities, while the tobacco price policy reduced socioeconomic inequalities. This is supported by the argument that some public health interventions may increase inequalities . “Upstream” interventions such as reducing price barriers are more likely to have positive effects on alleviating inequalities compared to “downstream” interventions to focus on individual-level factors such as information provided through education. Based on the review of the effects of public health policies on health inequalities, Thomson et al.  concluded that two types of oral health interventions had positive effects on inequalities: water fluoridation  and a national tooth brushing education campaign . Another study pointed out that dental insurance is an important driver for dental service use, as tackling financial barriers mostly reduces unmet dental needs [17, 43]. US studies to examine the effects of the Children’s Health Insurance Program also reported an increase in sealant treatment, fluoride tablets, and dental visits and a decrease in untreated caries since 1997, especially in children from lower-income households who benefited from free or subsidized school lunch programs .
Our study showed differential impacts of coverage expansion on dental health inequality between children and adolescents; the alleviation of inequalities was more salient among children while not among adolescents. What could explain such a difference? First, inequality may worsen as children grow older, as shown in previous studies [45–48]. A study based on the United Kingdom Millennium Cohort showed relatively narrow in health inequality when the children were younger (aged 3–5) . They were born when the New Labor Government introduced a sustainable strategy to address health inequalities. It could be inferred that the use of preventive dental services may alleviate the disease. Even the same intervention could not have the same effect on older children to alleviate inequality. The second possibility is the ‘inverse care law’ of public health care [49–52]. In the early stage, public health care is used by people with more resources such as information, time, availability, or money, which leads to deepening inequalities. The NHIS dental care coverage was implemented in December 2009, and it covered only children aged 6–14 years old for the first molar in permanent dentition with a 30% copayment. In 2012, the coverage was expanded to the second molar and adolescents up to 18 years old in 2013. It means that the adolescents aged 12–18 in the sixth wave were 6–12 years old in the fourth wave (2007–2009). A part of them was not eligible for the service because of age limitations until 2013. Later the service was available to all, but some of them already had or had experienced caries, in which dental sealants were no longer applicable. Third, as McLaren pointed out, sometimes the population strategy of prevention will not be effective in narrowing socioeconomic inequalities in health . Preventive services such as sealant treatment could inhibit dental caries, but it is not guaranteed to reduce socioeconomic inequalities in oral health . Based on Taiwan’s National Health Insurance Research Database, Hsu et al. showed that including the preventive provision of fluoride has an effect, but only for specific groups of children who are vulnerable to dental problems . Even though the percentage of children receiving fluoride was increasing, visits for dental caries decreased among those with highly severe caries of primary dentition. One UK study also showed that socioeconomic inequality remained despite no difference in dental health utilization . In the United States, income-related inequality in untreated dental caries among children has been steady over three decades since the 1970s [46, 47]. More salient inequality in dental health observed in developed countries rather than developing ones  may be associated with accessibility to dental treatment services as well as sugar consumption .
This study analyzed data from KNHANES, which is a yearly repeated cross-sectional survey and the data for every three years represent a different wave. The survey continues for three years, which offers the advantage of reflecting fast-changing disease patterns. There could be slight differences every year, meaning that it is imperative that data are handled carefully . This was the reason to analyze this data to compare before and after the policy implementations such as natural experiments. It can be assumed that the changes of Korean people could be found by this sample. Another advantage of this survey is carefully designed to be representative of national non-institutionalized civilians in South Korea. Well-trained dentists took part in this survey which makes the result stronger and reliable. The survey is repeated every year with different samples, not like a cohort. It could be a strength of this survey because the cohort might be impossible to reflect the change of the caries trends with representative samplings. The different characteristics of the fourth and sixth wave samples were applied to the data set with caries and sealant status changes.
There are several limitations of this study related to the coverage of dental sealants. The NHIS policy changes too often in relation to dental sealant treatment. From the 2010s, the government just agreed on an extension of the coverage provided for dental care to include preventive treatment for the first time. This has not been implemented before, as the government was wary of the possible financial burden. However, their expectations proved to be inaccurate, as fewer than 10% of children received the dental sealant service under the policy coverage every year. Because it was a new approach, the policy went through a transitional phase concerning the extended coverage of dental service in the beginning. This means that the change in policy could have affected the children and adolescents in this study unevenly. Later, in 2017, the government reduced the out-of-pocket payment from 30–10% of the total fee. This limitation can be overcome if monitoring of next wave study samples continues.