This is the first nationwide population-based, epidemiologic study of SLE among women of childbearing age in South Korea using NHIS-NHID that includes health information from nearly the entire Korean population. Our study found high incidence and prevalence rates of SLE in women during childbearing years, reflecting a high burden of disease in this particular population.
An average of 8.12 per 100,000 Korean women of childbearing age per year were at risk for developing SLE in this study. This average incidence was higher than the incidence reported previously in the total Korean population (3.72/100,000 person-years) and in Korean women of all ages (6.52/100,000 person-years), according to a study using information from the same database from the years 2005 to 2015 [7]. This suggests that women of childbearing age are a high-risk population for developing SLE. The incidence of SLE among Korean women of childbearing age was also higher compared with the overall incidence of SLE in the United States (US), Europe, and Asia. A recent epidemiologic review reported overall SLE incidences 5.2–7.2/100,000 person-years in the US [22–24], 1.0-4.9 in Europe [4, 5, 25], and 2.8–8.1 in Asia [21, 26–28].
A previous study reported a decrease in the incidence of SLE between 2005 and 2010 and then a slight annual increase until 2015 in Korea [7], but we observed a relatively constant rate of SLE incidence among Korean women of childbearing age. This suggests that the risk of developing SLE for the total population in a given year is affected by extrinsic factors including differences in environmental exposures such as an influenza epidemic, whereas the risk of developing SLE for the specific sociodemographic group of women of childbearing age is mainly affected by intrinsic factors such as age, gender, and female sex hormones [29, 30].
Although there was a slight bimodal pattern in incidence by age, the peak age of incidence in this study was between 25–39 years, with a lower incidence rate in the early (20–24 years) and late (40–44 years) childbearing age periods, suggesting that women are at risk of developing SLE during the period of time when they are most likely to have children (25–39 years). These results are consistent with previous Koreans studies that have reported a peak age of incidence in the 30–39 year age ranges [7, 21]. The peak age of incidence for SLE in other countries varies: Taiwan and Spain have peak incidence at the age of 20–29, and in Norway, the peak incidence age is 16–29 [11, 31, 32]. However, the incidence of SLE increases steadily with age in the US, peaking in women over fifty [33]. This might be related to an increased frequency of late-onset SLE in the Caucasian population [34].
The average prevalence of SLE in women during their childbearing years was 77.07/100,000 person-years (95% CI 75.76–78.39). The prevalence of SLE in women of childbearing age was 3.6-fold higher than the prevalence in total Korean population (28.02/100,000 person-years), which was reported using the same database from 2005–2015, and 1.5-fold higher than the prevalence in Korean women of all ages (51.00/100,000 person-years) [7]. Most studies report higher prevalence rates in women than in the overall population. Although there is no prevalence data for women of childbearing age, the prevalence in women was much higher in the US (159.8/100,000) and Taiwan (179.4/100,000) compared to Korea [26, 35]. Higher prevalence rates of SLE in women of childbearing age may reflect higher incidences of SLE in this specific age group compared to their lifetime risk as shown in this study.
Although we found a year to year variation of SLE prevalence in the total Korean population with a maximum of 14.2/100,000 person-years [7], there was less variation in the prevalence among women of childbearing age, suggesting that childbearing age itself imposes a significant burden on women with SLE.
The prevalence of SLE increased with age in women during their childbearing years, with a peak prevalence in the 40–44 year of age ranges. This finding is consistent with other studies in the total population [7]. This age-specific prevalence trend in SLE reflects the fact that SLE is a chronic autoimmune disease that requires lifelong management [3]. A steep increase in prevalence that plateaus in the 30–34 age group appeared to be related to a higher incidence between the ages of 25–39 years compared to the incidence in the 20–24 and 40–44 age groups. The plateau in prevalence of the 30–34 age group also coincides with the peak age for childbirth in Korea [36]. This suggests that Korean women of childbearing years have to face the burden of SLE through their pregnancies and childbirth.
Despite these important findings, this study has some limitations. First, since the NHIS-NHID are collected for insurance claim purposes only, the diagnostic codes may be inaccurate. To minimize this problem, we only included NHIS data starting from 2009, since all SLE patients were registered with an accurate diagnostic code to receive benefits from the government on a newly launched individual co-payment program for rare and intractable diseases which started in the second half of the year of 2008. Second, as NHIS data is limited to information on patients who visited health institutions, the prevalence and incidence rates might be underestimated. However, such an underestimation may have only slightly impacted our results, since the NHIS covers the entire population of Korea. Nevertheless, this is the first study defining incidence and prevalence rates for SLE among women of childbearing age, a unique population group with a high disease burden.