Autoimmune inflammatory rheumatic diseases (AIIRDs) mainly affect the musculoskeletal system and connective tissues due to altered immune regulation [1]. Several diseases, including rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), ankylosing spondylitis (AS), Behçet’s disease (BD), Sjögren’s syndrome (SjS), systemic sclerosis (SSc), idiopathic inflammatory myositis (IIM), polymyalgia rheumatica (PMR), and gout are included in this classification [2]. Infection is often considered a major trigger for AIIRD [3]. In December 2019, a novel coronavirus infection (coronavirus disease 2019 [COVID-19]) was identified, and it has rapidly spread globally [4]. Nonpharmaceutical interventions (NPIs) for COVID-19 have been reported to reduce the prevalence of Kawasaki disease and the rates of hospitalisation for respiratory and chronic lung diseases [5–7]. Conversely, the COVID-19 pandemic has been associated with an increased incidence of gestational diabetes mellitus, perforated appendicitis, depression and anxiety [8–10].

In the light of this and the potential mechanisms by which COVID-19 could influence the incidence of AIIRDs, this study aimed to determine if the COVID-19 pandemic was associated with changes in the reported incidence of AIIRDs. We extracted the incidence of AIIRDs in South Korea during the COVID-19 pandemic from a countrywide healthcare claims database and compared it with data from the previous 4 years.

South Korea initially gained attention in terms of the global COVID-19 response after the first COVID-19 patient was identified on January 20, 2020 [11]. In 2020, the number of cumulative confirmed COVID-19 cases in South Korea was 1.3 per thousand persons, which was much lower than that in other nations like the United States and the United Kingdom [12]. Nonetheless, the annual incidence of IIM, SLE, and BD decreased during the COVID-19 pandemic in 2020, according to this nationwide survey.

Several contributing factors can explain the reduction in the incidence of some rheumatic diseases during the COVID-19 pandemic. First, the incidence of rheumatic diseases may have temporarily decreased due to the decrease in healthcare utilisation driven by concerns about COVID-19 infection. During the COVID-19 pandemic in the United States, hospital admissions and emergency department visits were low, presumably related to a fear of COVID-19 infection [13–15]. A recent study demonstrated that the number of breast cancer screening, diagnosis and surgery cases decreased with the decrease in hospital visits, whereas the proportion of patients diagnosed with advanced breast cancer increased [11]. Essentially, according to the statistics of the Korean Health Insurance Review and Assessment Service, the total number of patients in 2020 was significantly lower than the average of the previous 4 years [16]. The monthly incidence of numerous diseases in the first half of 2020, the early phase of the COVID-19 pandemic, was much lower than that in the second half in the ARIMA analysis. It could be interpreted that the marked decrease in hospital visits in the early phases of the COVID-19 pandemic had been complementarily reversed in the second half of 2020. However, because the annual incidence of gout and hypertensive diseases did not significantly decrease in 2020 compared with before the COVID-19 pandemic, it is difficult to attribute the decrease in the annual incidence of IIM, SLE, and BD to a reduction in healthcare utilization. Second, South Korea has been thoroughly practising personal hygiene and the universal use of masks since the early days of the COVID-19 outbreak [17]. Furthermore, it strictly implemented social distancing during 2020 [18]. According to reports, these COVID-19 NPIs reduced the incidence of Kawasaki disease and the rates of hospitalisation due to respiratory and chronic lung diseases in South Korea [5,6,19]. A study using German national surveillance data also reported a decrease of respiratory and blood-borne infectious diseases, including measles and hepatitis C virus, owing to the implementation of NPIs during the COVID-19 pandemic [7]. Various viral infections are implicated in the development of autoimmune disorders through molecular mimicry and other mechanisms [3]. In particular, Epstein–Barr and hepatitis C viruses may promote uncontrolled autoantibody synthesis, resulting in AIIRDs [20]. Thorough implementation of the NPIs to prevent the spread of COVID-19 in South Korea reduced the incidence of several viral infections, which may have affected the incidence of AIIRDs.

The present study has some limitations. First, the National Health Insurance claims database lacked accurate clinical information on individual patients, such as laboratory and radiographic findings as well as lifestyle factors. Second, the disorders were identified using the registered diagnosis code, which may not accurately reflect the disease severity. Third, 1 year is too short to sufficiently evaluate the changes in the incidence of various chronic diseases, including most rheumatic diseases, in association with changes in societal and environmental factors. Thus, longer-term studies in more countries are needed in the future to better elucidate the potential link between the COVID-19 pandemic and various chronic diseases. The main strength of this study is that it is the first to report a significant decrease in the annual incidence of some rheumatic diseases during the COVID-19 pandemic, based on a nationwide claims database.

In conclusion, this study showed a significant decrease in the incidence of IIM, SLE, and BD during the COVID-19 pandemic in 2020, suggesting a potential link between societal and environmental changes associated with the COVID-19 pandemic, especially NPIs, and a decrease in the incidence of certain rheumatic diseases.