Introduction

Previous studies have suggested that the global prevalence rate of rheumatoid arthritis (RA) has remained consistent among decades1. However, in South Korea, several studies have reported a decrease in the prevalence of RA before the COVID-19 pandemic, followed by an increase during the pandemic2. They only provided information on RA prevalence from 2016 to 2020, where the impact of the pandemic to RA has not been reviewed. For this reason, we conducted a study on the prevalence trends of RA and OA between 1998 and 2021. Similar trends have been observed in the prevalence rate of osteoarthritis (OA) globally, with an increasing trend; however, in South Korea, it decreased before the pandemic and increased during the pandemic3,4.

Social distancing policy especially forcing to close the public gym facilities during the COVID-19 pandemic influenced domestic health concern2,5,6. Moreover, changes in physical activity, dietary habits, and mental health due to the COVID-19 restrictions may have influenced these prevalence rates7. However, further analysis and research are needed to determine the direct relationship between RA, OA, and the COVID-19 pandemic. Furthermore, some medications used to treat RA, such as corticosteroids and disease-modifying antirheumatic drugs, may suppress the immune system and increase the risk of infection8, thereby, making the patients more vulnerable to COVID-19 and its complications8. Therefore, it is crucial for patients with RA and COVID-19 to prioritize their health management9.

This study aimed to investigate the long-term trends in the prevalence of RA and OA using a representative large-scale dataset from 1998 to 2021. Additionally, we analyzed the specific social strata that were vulnerable during the COVID-19 period. Furthermore, our research findings may provide valuable information for developing policies for the treatment and prevention of these conditions following the COVID-19 pandemic.

Methods

Patient selection and data collection

This study utilized data from the Korea National Health and Nutrition Examination Survey (KNHANES) conducted between 1998 and 2021 by the Korea Centers for Disease Control and Prevention Agency (KDCA)10,11. The study population included adults aged ≥ 19 years, and the collected data included information on age, sex, residence, body mass index (BMI), education level, income, alcohol consumption, smoking status, and history of RA and OA12. We conducted our research focusing on the adult population, which ranges above 19 years in South Korea. A nationally representative sample of 163,221 participants was used to investigate the prevalence of RA and OA before and during the COVID-19 pandemic. The survey was conducted over 24 years and the number of participants surveyed in each year group was as follows: 51,515 in 1998–2001; 26,996 in 2005–2007; 20,070 in 2008–2010; 17,601 in 2011–2013; 17,129 in 2014–2016; 18,469 in 2017–2019; 5839 in 2020; and 5,602 in 2021.

The research protocol was approved by the Institutional Review Board of Kyung Hee University (KHUH 2022-06-042) and KDCA, and all participants provided written informed consent. Moreover, the KNHANES offers accessible public access to its data, which can be utilized as a valuable resource for diverse epidemiological investigations. This research adhered to the ethical guidelines established by relevant national and institutional review boards for human research and followed the 1975 Helsinki Declaration, as amended in 2008.

Ascertainment of RA and OA

The objective of our study was to investigate the risk factors related to the two most common types of arthritis, RA and OA, over a period of 24 years from 1998 to 2021. To achieve our research objective, we surveyed a large sample of participants and asked them the question: “Have you ever been diagnosed with RA or OA by a doctor?” Based on their answers, we categorized the participants into three groups: RA, OA, and both13. We collected data on various potential risk factors associated with the development of RA and OA, such as age, sex, lifestyle habits, and socioeconomic status. We conducted statistical analyses to examine the associations between these risk factors and the development of RA and OA and to identify any patterns or trends that emerged over 24 years.

Covariates

Covariates included age (19–29, 30–39, 40–49, 50–59, 60–69, 70–79, and ≥ 80 years), sex, region of residence (urban and rural)14,15,16, BMI group, household income (lowest, second, third, and highest quartile), education level (elementary school or lower, middle school, high school, and college or higher education), alcohol consumption (1–5 days/month, ≥ 6 days/month, and non-drinker), and smoking status (non-smoker, ex-smoker, and smoker). BMI was categorized into underweight (< 18.5 kg/m2), normal weight (18.5–22.9 kg/m2), overweight (23–25 kg/m2), and obese (≥ 25.0 kg/m2) according to the Asian-Pacific guidelines17,18.

Statistical analyses

The results of this study were presented using qualitative data expressed as proportions or percentages. Weighted multivariate regression model analyses were conducted to compare the estimates of each related factor before and during the COVID-19 pandemic, with weighted odds ratios (ORs) with 95% confidence intervals (CIs)19. The prevalence of RA and OA was calculated using data from the KNHANES, spanning from 1998 to 2021, stratified by year group. Weighted complex sampling analysis was performed to ensure accurate estimation. Binomial or linear logistic regression models were used to compute the ORs with 95% CIs or β-coefficients with 95% CIs. To ensure robustness of the main findings, a stratification analysis was performed using variables such as sex, educational level, region of residence, and income in all the regression models. Furthermore, the ratio of ORs was calculated to estimate the interaction term of each risk factor and identify groups that were more vulnerable to the patient with RA and OA during the pandemic. Overall, this study aimed to provide a comprehensive analysis of the impact of the COVID-19 pandemic on the prevalence of RA and OA and to identify the factors that contribute to vulnerability to these conditions. The SAS software (version 9.4; SAS Institute, Cary, NC, USA) was used for statistical analyses, with a two-sided test, and a p-value ≤ 0.05 was considered statistically significant19.

Results

We recruited 163,221 participants from 1998 to 2021 with the following distribution of characteristics: age (19–29 years, 18.18% [95% CI 17.82 to 18.53]; 30–39 years, 20.88% [20.47 to 21.29]; 40–49 years, 21.70% [21.35 to 22.06]; 50–59 years, 17.85% [17.56 to 18.14]; 60–69 years, 11.88% [11.64 to 12.12]; 70–79 years, 7.38% [7.19 to 7.58]; and ≥ 80 years, 2.13% [2.03 to 2.23]) and sex (male, 49.39% [49.14 to 49.64] and female, 50.61% [50.36 to 50.86]). These results are presented in Table 1.

Table 1 General characteristics of Korean adults, in the data obtained from the KNHANES from 1998 to 2021 (n = 163,221).
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Table 2 illustrate the prevalence of RA, OA between pre-pandemic and during the pandemic. The prevalence of RA and OA showed a U-curve between pre-pandemic and during the pandemic (RA: from 1.46% [95% CI 1.38 to 1.54] in 2005–2019; 1.23% [0.92 to 1.54] in 2020; and 1.36% [1.02 to 1.69] in 2021, OA: from 7.68% [7.49 to 7.88] in 2005–2019; 8.04% [7.18 to 8.89] in 2020; and 8.27% [7.35 to 9.19] in 2021) (Table S1).

Table 2 National trends of the prevalence of RA and OA before and during the COVID-19 pandemic, weighted % (95% CI), in the data obtained from the KNHANES.
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Table 3 and Fig. 1 show the national trends in the prevalence of overall RA and OA, as well as the prevalence of RA and OA separately, along with the β-coefficients of the ORs before and during the COVID-19 pandemic. A statistically significant decrease in the weighted prevalence of RA was observed pre-pandemic (2005–2007, 1.91% [95% CI 1.72 to 2.10]; 2008–2010, 1.79% [1.59 to 1.99]; 2011–2013, 1.40% [1.23 to 1.58]; 2014–2016, 1.49% [1.31 to 1.67]; 2017–2019, 1.55% [1.36 to 1.75]; and 2020, 1.23% [0.92 to 1.54]), whereas a slight increase was observed during the pandemic (2020, 1.23% [0.92 to 1.54] and 2021, 1.36% [1.02 to 1.69]). Similarly, decrease in the weighted prevalence of OA was observed before the pandemic (2005–2007, 9.75% [9.29 to 10.20]; 2008–2010, 8.60% [8.12 to 9.08]; 2011–2013, 7.97% [7.51 to 8.44]; 2014–2016, 8.06% [7.57 to 8.56]; 2017–2019, 8.42% [7.91 to 8.93]. A slight increase was observed during the pandemic (2020, 8.04% [7.18 to 8.89] and 2021, 8.27% [7.35 to 9.19]). Similar patterns and trends were observed for the prevalence of OA and RA stratified by age, sex, region of residence, education level, and household income (Table S2). While investigating the weighted OR in 2021 with respect to that of 2020 (reference), there was a surge regarding RA among underweight group (OR, 6.62 [1.28 to 34.36]).

Table 3 National trend of the RA and OA prevalence and β-coefficients of the odds ratios before and during the COVID-19 pandemic, weighted % (95% CI), in the data obtained from the KNHANES.
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Figure 1
figure 1

Seventeen-year trends in the prevalence of rheumatoid arthritis and osteoarthritis in South Korea, 2005–2021. OA osteoarthritis, RA rheumatoid arthritis.

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Table 4 shows the pandemic-related effects on vulnerable groups of patients with RA and OA. A statistically significant difference was observed among vulnerable individuals with OA as follows: participants ≥ 60 years old (ratio of ORs 1.222 [95% CI 1.011 to 1.477]), urban residents (ratio of ORs 1.289 [1.007 to 1.650]), and participants with a high level of education (ratio of ORs 1.360 [1.119 to 1.653]). In contrast, no significant difference in pandemic-related effects was observed among vulnerable individuals with RA (Table S3).

Table 4 Difference between pre- and during the COVID-19 pandemic by the ratio of ORs on OA and RA, weighted % (95% CI), in the data obtained from the KNHANES.
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Discussion

Key results

The present study examined trends in the prevalence of RA and OA over a 24-year period from 1998 to 2021 and evaluated the differences in prevalence before and during the COVID-19 pandemic (n = 163,221). The prevalence of RA and OA consistently declined before the onset of the pandemic, but there was a slight increased during the pandemic. However slight increasing prevalence during pandemic was not statistically significant. Notably, patients with OA had a significantly high prevalence among vulnerable groups, including individuals aged ≥ 60 years, urban residents, and those with a high education level. Therefore, these findings suggest policy researchers should develop personalized policy proposals to address the needs of these groups during the pandemic.

Global epidemiology and mechanism

Previous studies have reported a global increase in the prevalence of RA and OA, before the COVID-19 pandemic20. However, in South Korea, the prevalence of both conditions has consistently declined, contrary to global trends21. Similar trends of decline in the prevalence of RA have been observed in Japan and Sweden as well22. A Japanese study found a correlation between a decrease in RA prevalence and changes in lifestyle factors, such as dietary habits and smoking, while a Swedish study has linked this decrease to improvements in medical accessibility for patients with RA16,23. Although we cannot accurately identify the specific reasons for the variation in RA prevalence across countries, we believe that factors, such as genetic, cultural disparities, and health level may have a significant impact24. In addition, increased BMI and muscle weakness are closely related to OA and RA, as these diseases are associated with factors that contribute to joint strain25,26.

During the COVID-19 pandemic, the social distancing policies and the closure of sports facilities significantly diminished the public health standards, which could generally explain the observed increase in the prevalence of RA and OA27,28.

RA is a complex disease with multiple factors contributing to its development, with genetics potentially being a significant factor29. Other studies have reported that numerous variables can affect the prevalence of RA30. Therefore, further research is warranted to investigate this matter thoroughly. An increase in the prevalence of OA and RA was the most prominent in the highly educated older adult population living in urban areas31. These individuals who were retired and had reduced health standards due to COVID-19 were most affected32. According to previous research, the domestic standard of physical activity among South Korean adult population exhibited a declining trend until 2017, followed by an upturn until 201933. In 2020, confronting the pandemic, there was a significant decline in physical activity. This pattern closely aligns with the prevalence trends in RA and OA, which could substantiate our hypothesis.

This finding suggests that OA may occur when physical activity falls below a certain threshold, which warrants further investigation. It is crucial to recognize this issue at a national level and implement policies to encourage exercise among the older adults, prevent the occurrence of OA, and provide the appropriate treatment for those with the disease.

Strengths and limitations

The present study is significant in utilizing population-based nationwide representative data to compare the trends and prevalence of RA and OA, both before and during the COVID-19 pandemic34. The use of national data is particularly noteworthy, as individual data collection during a pandemic could have been challenging. Furthermore, this study’s utilization of survey data collected from 1998 to 2021, spanning a period of 24 years, adds academic value to the research findings due to its long-term nature. However, this study has several limitations. First, data were collected only from Koreans, thus limiting generalizability of the results to other ethnic groups or countries. Further research with multiple ethnicities and different countries is necessary to examine the global impact of the COVID-19 pandemic35. Second, data on patients with separate covariate of RA and OA were only available from 2005–2021and information prior to 2005 was not available. Third, some variables in the study, such as height and weight, were self-reported, which may have introduced bias. To address this concern, the KDCA should include a question in the survey about when participants' heights and weights were last measured36. In addition, the diagnosis of RA and OA was self-reported, potentially resulting in a recall bias. Lastly, we substituted physical activity data from the previous study, since we were unable to conduct our own analysis of domestic physical activity levels.

Conclusion

Our study identified long-term trends in the prevalence of RA and OA spanning 24-years from 1998 to 2021, especially focusing on the COVID-19 pandemic. The results showed a consistent decline in the prevalence of both RA and OA until 2016 followed by an oscillation until 2021, where significant drop occurred in 2019. Notably, OA exhibited a higher prevalence among vulnerable groups, such as individuals over 60 years of age, urban residents, and those with a high education level. When investigating the RA prevalence trend, there was no specific vulnerable population. This outcome will help government policy researchers devise personalized healthcare policies targeting the vulnerable groups. While our study did not find any evidence of a relationship between the COVID-19 pandemic and the prevalence of RA, additional follow-up studies is needed to further explore this topic.