This cohort study describes the incidence of Kawasaki disease overall across 50 years and by age groups to assess whether epidemiologic patterns changed after the COVID-19 pandemic.
Key Points
Question
Following an initial decline in the incidence of Kawasaki disease (KD) during the COVID-19 pandemic, did the epidemiology of KD change after the relaxation of pandemic restrictions in Japan?
Findings
In this cohort study of 29 841 patients, KD incidence demonstrated age-dependent changes after the COVID-19 pandemic. Infants showed minimal changes, whereas children 5 to 9 years of age showed greater changes, suggesting different exposure pathways to potential KD triggers.
Meaning
Results from this study suggest that KD pathogenesis research should consider an age-stratified approach, distinguishing infants from older children.
Abstract
Importance
The COVID-19 pandemic may serve as a natural experiment, with pandemic-associated changes in the epidemiology of Kawasaki disease (KD) potentially informing understanding of KD pathogenesis.
Objectives
To assess long-term KD incidence across 50 years and to examine pandemic-associated changes in KD epidemiology across age groups.
Design, Setting, and Participants
This cohort study was a descriptive epidemiologic analysis of data from the Japanese Nationwide Survey of Kawasaki Disease conducted from 2023 to 2024, along with 50 years of historical data. Data were included from patients with KD in the current 2023-2024 survey (n = 29 841) and historical and current survey data from 1975 through 2024 (n = 467 456) for trend analysis.
Main Outcomes and Measures
Annual KD incidence overall and stratified by 3 age groups: infants (<1 year), 1 to 4 years, and 5 to 9 years.
Results
The current survey registered 15 032 and 14 809 patients with KD in 2023 and 2024, respectively (overall, 57.3% male; median [IQR] age, 2 [1-4] years). Across the 50-year study period, despite the population under 5 years of age declining in Japan by 61.6%, KD incidence rates increased 16.1-fold. Across the COVID-19 pandemic period, patient numbers reached a nadir of 10 333 in 2022, then rebounded to 15 032 cases (45.5% increase) in 2023. Notably, decline and rebound patterns differed by age. Indexed incidence rates (using 2017 as a baseline of 100) demonstrated that patients 5 to 9 years of age had the largest decline (nadir of 54.2 in 2021, 45.8% below baseline) and the largest rebound (134.8 in 2024, 34.8% above baseline). In contrast, patients younger than 1 year showed the smallest decline (nadir of 73) and incidence rates did not return to baseline levels. When comparing the magnitude of rebound in KD incidence, a distinct age-dependent dose-response pattern was observed: 26.1% for infants, 38.9% for patients aged 1 year, 42.7% for patients aged 2 years, 85.8% for patients aged 3 years, 105.3% for patients aged 4 years, and 166.7% for patients aged 5 to 9 years.
Conclusions and Relevance
In this nationwide cohort study of KD in Japan, KD incidence rebounded after the relaxation of COVID-19 pandemic-related restrictions, with age-dependent patterns. Nonpharmaceutical interventions (eg, mask-wearing) during the pandemic may explain the greater rebound pattern observed in older children; however, the minimal changes in infants suggest different exposure pathways to potential KD triggers. These findings support an age-stratified approach to KD pathogenesis research, distinguishing infants from older children.
Introduction
Kawasaki disease (KD) is an acute febrile systemic vasculitis that predominantly affects children under 5 years of age, involving medium-sized arteries and multiple organs, with coronary artery lesions (CALs) as the major complication. Since Dr Tomisaku Kawasaki first described KD in 1967,1,2 although substantial evidence has accumulated for the management and prevention of CALs,3,4,5,6,7,8,9,10 the underlying cause of this mysterious disease has eluded global researchers for more than 5 decades.11 However, changes in the epidemiology of KD during the COVID-19 pandemic provide insights into its potential etiology.
During the COVID-19 pandemic, KD incidence declined worldwide.12,13,14,15,16,17,18,19,20 In Japan, the percentage of patients with KD decreased by 36% during 2020 compared with the previous year, with different reduction patterns observed between children younger than 1 year and older children, supporting the hypothesis of a potential KD pathogenesis involving transmission among children.15 In the postpandemic period, following the lifting of global mitigation measures in 2023, several studies reported a large rebound in the number of patients with KD.19,20 However, these studies were limited to regional cohorts with restricted sample sizes. Comprehensive nationwide data on the postpandemic epidemiology of KD remain limited.
The Japanese Nationwide Survey of Kawasaki Disease (JNSKD), with over 50 years of history, covers an estimated 94% of KD cases diagnosed throughout Japan.21 The current survey, completed in 2025, collected data on all patients with KD diagnosed from 2023 through 2024. Using these survey data, this study assessed long-term KD incidence across 50 years and examined pandemic-associated changes in KD epidemiology across age groups.
Methods
JNSKD
The Jichi Medical University Clinical Research Ethics Committee approved this cohort study and waived the requirement for informed consent because the study used existing, anonymized data and did not involve any intervention or interaction with participants. The study followed the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) reporting guideline.
The JNSKD has been conducted biennially since 1971 to collect information on patients diagnosed with KD during the preceding 2 years throughout Japan.15,22,23 The survey methodology remained unchanged from the first survey through the survey in 2023. Traditionally, survey respondents included hospitals with 100 or more beds and a pediatric department and specialized pediatric hospitals with fewer than 100 beds. The current survey updated these 2 traditional inclusion criteria to maximize survey coverage given that some hospitals treating patients with KD no longer meet the criterion of 100 of more beds due to reduced hospital capacity, whereas other hospitals no longer treat patients with KD due to changes in clinical practice associated with the decreasing pediatric population. For the present study, we added the inclusion criterion of hospitals reporting at least 1 patient with KD in the previous 2 surveys, conducted in 2021 and 2023. We also established the following exclusion criteria to exclude hospitals that did not treat acute illnesses such as KD: (1) specialized pediatric rehabilitation facilities; (2) facilities offering only specialized pediatric outpatient services; and (3) hospitals that withdrew from the previous survey.
Using these inclusion and exclusion criteria, 1589 hospitals were identified as eligible survey respondents. All respondents provided information on patients diagnosed with KD from January 1, 2023, through December 31, 2024. For respondents reporting at least 1 case, patient information was collected using either paper-based or electronic data collection methods via the REDCap (Research Electronic Data Capture)24 platform. To ensure data quality, incomplete or unclear responses were reviewed, and respondents were contacted directly for clarification. Additionally, when the same patient was reported from multiple hospitals (eg, due to hospital transfer), duplicate records were identified and merged into a single case. Survey data collection and processing were completed by October 2025.
Diagnosis of KD
The diagnosis of KD was based on the current Japanese diagnostic guideline, revised in 2019.25 The 6 principal signs of KD are fever, bilateral conjunctival injection, oral mucosal changes, polymorphous skin rash, peripheral extremity changes, and nonsuppurative cervical lymphadenopathy. The diagnostic type was binary and classified as complete KD (5 or 6 principal features) or incomplete KD (fewer than 5 principal features).3,25
Measurements
Basic epidemiologic information included age, sex, and the date and day of illness at the first hospital visit. The season was categorized based on the month of the first hospital visit: winter (December through February), spring (March through May), summer (June through August), or autumn (September through November). The first day of illness was defined as the day of KD onset.
Clinical characteristic information included diagnostic type, treatment modalities, and CALs. Treatment modalities included initial intravenous immunoglobulin (IVIG) administration, steroid combination (for intensification of initial IVIG), and additional IVIG administration. The day of illness at initial IVIG administration was recorded. CALs were recorded at the first hospital visit26,27 and in the postacute period (approximately 30 days after KD onset), with the latter recording defined as coronary artery sequelae.15 CALs were evaluated using either z scores28,29,30 or the Japanese Circulation Society criteria.28
Statistical Analysis
Previous studies found differences in KD incidence reduction patterns between younger and older patients with KD during the early and postacute COVID-19 pandemic period (2019-2022).15,18 Building on these findings, the present study focused on age-related differences in incidence as well as epidemiologic and clinical characteristics. To provide historical context, we used data from past nationwide surveys in addition to the newly collected data from the current survey.
Patient Numbers and Incidence Rates Across 50 Years
First, annual numbers of patients with KD over a 50-year period from 1975 to 2024 were summarized. Patients were classified into 3 age groups (<1 year, 1-4 years, and ≥5 years). Additionally, the annual proportion (percentage) of patients aged 5 years or older was calculated. Second, age-specific incidence rates were determined by dividing the annual number of patients by the corresponding population for each age group (per 100 000 age-specific children per year). Population data were obtained from national vital statistics for Japan.31 Patients were categorized into 3 age groups (<1 year, 1-4 years, and 5-9 years), and patients 10 years or older (<1% of all cases) were excluded. The temporal changes in both population size (0-4 years and ≥5 years) and incidence rate were illustrated.
Pandemic-Associated Changes in KD Epidemiology
Using data from the current survey (2023-2024) as a reference, we compared epidemiologic and clinical characteristics across 3 pandemic periods: before (January 1, 2017, to December 31, 2019), during (January 1, 2020, to December 31, 2022), and after (January 1, 2023, to December 31, 2024). Year 2023 was defined as the start of the period after the pandemic, based on Japan’s reclassification of COVID-19 under the Infectious Disease Control Law and the concurrent World Health Organization declaration ending the COVID-19 public health emergency.
We first assessed temporal changes in incidence rates across the 3 periods using an indexed chart, with 2017 set as the baseline year (index = 100). Patients were categorized into 1 of 3 age groups (<1 year, 1-4 years, and 5-9 years). Second, seasonal distribution and clinical characteristics (days of illness at first hospital visit, diagnostic type, treatment modalities, and CALs) were compared across the 3 pandemic periods. We used χ2 tests to compare proportions between the periods after vs before and during. Finally, the magnitude of the postpandemic rebound in incidence was quantified by age (0, 1, 2, 3, 4, and 5-9 years). For each age group, the percentage change from minimum to maximum incidence from 2021 through 2024 was calculated.
Patient characteristics in the period from 2023 through 2024 were separately analyzed by age group (0-4 vs ≥5 years). All categorical variables are presented as percentages. Given the large sample size, a more stringent statistical significance threshold of a 2-sided P < .01 was applied. All analyses were performed using IBM SPSS Statistics for Windows, version 30 (IBM Corporation).
Results
Of the 1589 eligible hospitals nationwide, 1272 (80.1%) responded, providing information on 29 841 patients diagnosed with KD from January 1, 2023, to December 31, 2024 (15 032 in 2023 and 14 809 in 2024); 12 730 patients (42.7%) were female, 17 111 patients (57.3%) were male, and the median (IQR) age was 2 (1-4) years. Additional demographic information and patient characteristics are presented in the eFigure and eTable 1 in Supplement 1. Including these patients identified in the current survey, the database was updated to a total of 475 529 patients registered since the first survey in 1971. The following analyses were conducted using specific subsets extracted from this database.
Patient Numbers and Incidence Rates Across 50 Years
Figure 1 shows the 50-year historical data for 467 456 patients with KD from 1975 to 2024. During the first 25 years (1975-1999), 3 epidemic peaks were observed, in 1979, 1982, and 1986. In the latter 25 years (2000-2024), patient numbers increased from 8267 cases in 2000 to a peak of 17 364 cases in 2018. Following a nadir of 10 333 cases in 2022, patient numbers rebounded to 15 032 cases in 2023 (an increase of 4699 cases [45.5%] from the previous year), followed by 14 809 cases in 2024. The percentage of patients 5 years of age or older (Figure 1B) ranged from 11.0% to 14.5% between 2000 and 2020 but reached a nadir of 9.7% in 2021. Subsequently, this percentage increased markedly, reaching 18.4% in 2024, the highest percentage in 50 years.
Figure 1. Bar and Line Graphs Showing the Annual Number of Patients With Kawasaki Disease and the Percentage of Those Patients Aged 5 Years or Older Across 50 Years, 1975 through 2024 (N = 467 456).
Patients registered in the database lacking age data (n = 660) were excluded from the analysis.
Age-specific incidence rates demonstrated distinct patterns during and after the COVID-19 pandemic (Figure 2). For patients 5 to 9 years of age, the incidence rate per 100 000 population reached a nadir of 19.9 in 2021, but subsequently rebounded, reaching the highest rate (53.3) recorded across the 50-year period in 2024. When focused on patients younger than 1 year, the incidence rate per 100 000 population reached a historical peak of 380.5 in 2019, declined from 2020 to 2021, and rebounded from a trough of 276.9 in 2022 to 352.9 in 2024; however, the 2024 rate did not recover to the 2019 peak level. Despite population declines of 61.6% for the age group 0 to 4 years (from 9.9 million in 1975 to 3.8 million in 2024) and 54.0% for the age group 5 to 9 years (from 10.0 million in 1980 to 4.6 million in 2024), KD incidence rates increased 16.1-fold and 19.0-fold, respectively, over the study period.
Figure 2. Line and Area Graphs Showing Age-Specific Population and Annual Incidence Rates of Kawasaki Disease Among Infants and Children Aged 1 to 4 Years and 5 to 9 Years Across 50 Years, 1975 through 2024.
Shaded area represents the total age-specific population in Japan for the respective age group; line chart, incidence rates calculated using the formula (annual number of patients/age-specific population) × 100 000. Patients registered in the database 10 years of age or older (n = 3961) or lacking age data (n = 660) were excluded from the analysis.
Pandemic-Associated Changes in KD Incidence
Figure 3 shows indexed incidence rates with 2017 as the baseline year (index = 100) for 3 age groups, which included 112 819 patients. During the pandemic period, all age groups experienced declines, but the decline differed by age. Patients 5 to 9 years of age demonstrated the largest decline, reaching a nadir of 54.2 in 2021 (45.8% below baseline), whereas patients younger than 1 year showed the smallest decline to 72.9 in 2022. After the pandemic period (2023-2024), recovery patterns also differed substantially by age. Patients 5 to 9 years of age demonstrated the largest rebound, surging to 134.8 in 2024 (34.8% above baseline). However, patients younger than 1 year did not return to baseline levels. Patients 1 to 4 years of age demonstrated an intermediate pattern, with moderate declines during the pandemic period and recovery to near-baseline levels in 2023 through 2024.
Figure 3. Line Graph of Age-Specific Incidence of Kawasaki Disease Before, During, and After the COVID-19 Pandemic.
For each age group, the incidence rate in 2017 was set as the baseline (n = 100). Rates for all other years were then calculated using the formula (incidence rate in that year/incidence rate in 2017) × 100.
Table 1 shows the magnitude of postpandemic rebound in incidence by age. The percentage change progressively increased with age, from 26.1% for patients younger than 1 year to 38.9% for 1 year of age, 42.7% for 2 years, 85.8% for 3 years, 105.3% for 4 years, and 166.7% for 5 to 9 years of age, indicating a distinct age-dependent dose-response pattern.
Table 1. Postpandemic Rebound in the Incidence of Kawasaki Disease by Age Among 51 051 Patientsa.
| Age group, y | Incidence rate per year, per 100 000 population | Percentage changeb | |||
|---|---|---|---|---|---|
| 2021 | 2022 | 2023 | 2024 | ||
| <1 | 299.4 | 279.9 | 335.8 | 352.9 | 26.1 |
| 1 | 415.2 | 344.8 | 456.5 | 478.9 | 38.9 |
| 2 | 283.4 | 241.7 | 345.0 | 331.0 | 42.7 |
| 3 | 155.4 | 150.0 | 278.8 | 250.7 | 85.8 |
| 4 | 89.4 | 88.1 | 180.1 | 180.8 | 105.3 |
| 5-9 | 20.0 | 23.6 | 46.4 | 53.3 | 166.7 |
Of 62 944 patients registered from 2021 through 2024, those 10 years of age or older (n = 11 893) were excluded from this analysis.
Calculated as [(maximum − minimum)/minimum] × 100 for each age group. The minimum and maximum values were in 2022 and 2024, respectively, except that the maximum was in 2023 for patients 2 and 3 years of age, and the minimum was in 2021 for patients 5 to 9 years of age.
When comparing clinical characteristics across the 3 pandemic periods (Table 2), significant differences were observed in several variables. For seasonal distribution, winter was the peak season of KD incidence in the periods before and during the pandemic (26.8% and 27.4%, respectively); however, summer predominated in the period after the pandemic (28.9%). Additionally, the percentages of patients receiving initial IVIG and steroid combination treatment were significantly higher in the period after the pandemic (96.6% and 15.1%, respectively), while the percentage receiving additional IVIG was lowest after the pandemic (21.4% vs 25.4% before and 27.0% during). Patients in the period after the pandemic (6.9%) received treatment significantly later (initial IVIG administration at ≥8 days of illness) than those before (5.6%) or during (5.7%) the pandemic. Nevertheless, no significant difference was found in the percentage of patients who developed CALs among the 3 periods (eg, percentage at first hospital visit, 4.0% before and during the pandemic and 3.8% after the pandemic).
Table 2. Comparison of Epidemiologic Features of Kawasaki Disease Before and During vs After the COVID-19 Pandemic.
| Characteristic | Patients, No. (%) by COVID-19 pandemic perioda | P valueb | |||
|---|---|---|---|---|---|
| Before (2017-2019) (n = 49 875) | During (2020-2022) (n = 33 103) | After (2023-2024) (n = 29 841) | After vs before the pandemic | After vs during the pandemic | |
| Age group, y | |||||
| <1 | 9459 (19.0) | 6989 (21.1) | 4934 (16.5) | <.001 | <.001 |
| 1-4 | 33 959 (68.1) | 22 323 (67.4) | 19 803 (66.4) | ||
| ≥5 | 6457 (12.9) | 3791 (11.5) | 5104 (17.1) | ||
| Season | |||||
| Winter (Dec-Feb) | 13 350 (26.8) | 9072 (27.4) | 6910 (23.2) | <.001 | <.001 |
| Spring (Mar-May) | 12 505 (25.1) | 8213 (24.8) | 8106 (27.2) | ||
| Summer (Jun-Aug) | 11 987 (24.0) | 8393 (25.4) | 8630 (28.9) | ||
| Autumn (Sep-Nov) | 12 033 (24.1) | 7425 (22.4) | 6195 (20.8) | ||
| Sex | |||||
| Female | 21 446 (43.0) | 14 048 (42.4) | 12 730 (42.7) | .35 | .57 |
| Male | 28 429 (57.0) | 19 055 (57.6) | 17 111 (57.3) | ||
| Days of illness at first hospital visit | |||||
| 1-4 | 31 663 (63.5) | 20 782 (62.8) | 17 428 (58.4) | <.001 | <.001 |
| 5-7 | 16 394 (32.9) | 11 024 (33.3) | 11 040 (37.0) | ||
| ≥8 | 1817 (3.6) | 1296 (3.9) | 1362 (4.6) | ||
| Complete KDc | 39 733 (79.7) | 26 498 (80.1) | 24 121 (80.8) | <.001 | .01 |
| Initial IVIG administration | 47 325 (94.9) | 31 724 (95.8) | 28 821 (96.6) | <.001 | <.001 |
| Steroid combinationd | 6365 (14.4) | 4304 (14.4) | 4341 (15.1) | <.001 | <.001 |
| Days of illness at administrationd | |||||
| 1-4 | 17 386 (36.8) | 13 156 (41.5) | 11 346 (39.4) | <.001 | <.001 |
| 5-7 | 27 278 (57.7) | 16 735 (52.8) | 15 468 (53.7) | ||
| ≥8 | 2625 (5.6) | 1822 (5.7) | 1999 (6.9) | ||
| Additional IVIG administration | 10 256 (25.4) | 7412 (27.0) | 6155 (21.4) | <.001 | <.001 |
| Coronary artery lesione | |||||
| At first hospital visit | 2000 (4.0) | 1330 (4.0) | 1135 (3.8) | .18 | .21 |
| In subacute phase (sequelae)f | 1096 (2.2) | 687 (2.1) | 600 (2.0) | .09 | .60 |
Abbreviations: KD, Kawasaki disease; IVIG, intravenous immunoglobulin.
Sample sizes and percentages for some variables vary because of missing data.
χ2 Tests comparing the period after the pandemic with the period before or during the pandemic.
Defined as cases with 5 or 6 principal clinical features of KD; all other cases were classified as incomplete KD.
Among patients who received initial IVIG administration.
Subtypes (small, medium, and giant coronary aneurysms) are presented in eTable 2 in Supplement 1.
Defined as abnormalities detected at approximately 30 days after KD onset.
Discussion
This cohort study using JNSKD data found that KD incidence in Japan, which had increased for 50 years before declining rapidly during the COVID-19 pandemic, rebounded promptly following the relaxation of pandemic-related restrictions. Notably, the decline and rebound patterns differed across age groups. Both the decline and rebound were more pronounced with increasing age: older children (5-9 years) showed the largest magnitude of change, whereas infants (<1 year) showed the least, indicating an age-dependent pattern.
Using data from the JNSKD survey covering 2019 through 2020, we previously found that nonpharmaceutical interventions (NPIs) implemented during the COVID-19 pandemic, such as mask-wearing and social distancing, were associated with a substantial reduction in KD incidence, with a greater reduction observed in older children.15 Those findings suggested a potential KD pathogenesis involving transmission among children.15 Under strict NPIs during the pandemic, pediatric respiratory infections also showed similar declines.32,33,34,35,36,37,38,39,40 Bardsley et al32 observed that respiratory syncytial virus (RSV) cases decreased by more than 99% in England during the winter of 2020 to 2021; RSV cases rebounded markedly after NPIs were lifted, with a 12-fold increase during the summer of 2021. The “immunity debt” hypothesis explains this response, proposing that children who missed pathogen exposure during the NPI period remained susceptible and developed infections with exposure after NPI relaxation.37,38,39,40 Notably, a dose-response relationship has been observed for immunity debt.40 Among 13 European countries, those with stricter NPIs experienced greater disease reduction during the pandemic, followed by greater rebound after NPI relaxation.40 Our findings align with this observation: the magnitude of the rebound progressively increased with age. Older children, presumably more responsive to NPIs, showed greater decline and rebound in KD incidence, consistent with a dose-response relationship. Furthermore, our findings showed shifts in KD seasonality across the pandemic period. Similar shifts in RSV seasonality have been observed and explained by the immunity debt hypothesis,32,33,34 suggesting that our KD findings may involve a similar mechanism.
However, unlike RSV and other respiratory infections, the immunity debt hypothesis may be tangential to explaining the rebound in KD incidence after NPI relaxation. This rebound may simply reflect a large population of genetically KD-susceptible children who had been protected from airborne exposure by NPIs, such as mask-wearing, without any immune involvement. This is consistent with previous studies proposing that KD may be triggered by unidentified respiratory pathogens.12,41,42 Since most children develop KD only once in their lifetime (although 4%-5% of patients experience recurrence15,22), an episode of KD may serve as an immunizing event against subsequent exposures. For these reasons, the “susceptibility accumulation” hypothesis may alternatively explain the rebound in KD incidence after NPIs were lifted. Assuming this alternative hypothesis, the rebound observed in 2023 through 2024 may be self-limiting, as the population of susceptible children diminishes over time. The next nationwide survey, covering data from 2025 onward, will be informative in testing this prediction.
The immunity debt hypothesis may not explain the minimal changes we observed in infant KD patients. Similar findings have been reported in other pediatric infectious diseases.33,35 Protection against RSV in early infancy depends on maternal antibodies.43 If pregnant women had reduced RSV exposure during the NPI period, their infants would be susceptible and develop infections on exposure after NPI relaxation. However, contrary to this expectation, a Korea nationwide study found that RSV hospitalization rates (per 100 000 children) among infants (<1 year) declined (from 1396 prepandemic to 1236 postpandemic), whereas those rates among children 1 to 6 years of age increased by 55% (from 231 to 358).33 Similarly, a study conducted in Italy showed that the age at RSV infection onset increased rather than decreased (from 2.7-2.9 months to 3.2 months).35 These findings suggest that the immunity debt hypothesis may not fully apply to infants. Consistent with these RSV findings, infants in our KD study showed minimal rebound compared with older children. We propose that this paradox may be explained by differences in pathogen exposure pathways between infants and older children. DeHaan et al44 found that KD epidemiology differed between infants and older children: seasonal cycles in older children were synchronized across geographic regions and associated with the school calendar, whereas infants showed no such synchronization, suggesting different pathways of exposure to KD triggers. The authors suggested that infants are primarily exposed to KD triggers within the household from parents or older siblings, whereas older children are exposed through social activities outside the home.44 Our findings are consistent with this interpretation. Older children, whose exposure depends more on social activities, showed greater reduction during the NPI period and greater rebound after NPI relaxation, whereas infants, who are mainly exposed within the household, showed minimal changes. DeHaan et al44 also proposed that wind-borne aerosols may carry KD-triggering agents, with infants being exposed indirectly through contact with family members, as multiple studies have reported associations between meteorologic factors and KD incidence.45 Collectively, our findings suggest that research focusing on KD pathogenesis should adopt an age-stratified approach, distinguishing infants from older children.
Limitations
This study has limitations. First, due to the observational study design, we could not assess a causal effect of NPIs on KD incidence. Second, we did not collect serologic data, which may have provided clues regarding the applicability of the immunity debt hypothesis. Third, echocardiographic assessment was not centralized. Pediatricians used either z scores or Japanese criteria, potentially leading to the underestimation of CAL detection.46 Fourth, with updated hospital selection criteria, 80.1% of eligible hospitals responded in the current survey (vs 74.6% in the last survey23), which may have affected higher patient numbers observed in the period after the pandemic; conversely, nonresponding hospitals may have introduced underreporting. Fifth, although the period during the pandemic (2020-2022) was defined as a single category for descriptive clarity, NPI intensity was not uniform across this period.15 Sixth, evaluation of rebound in incidence rates among age groups was descriptive, and year-specific fluctuations may have affected accurate quantification. Finally, because this study was based on patients in Japan only, findings might not be generalizable to populations outside Japan.
Conclusions
In this nationwide cohort study of KD in Japan, the incidence of KD demonstrated age-dependent changes after the COVID-19 pandemic. During and after the pandemic, decline and rebound patterns differed substantially by age. Infants showed minimal changes, whereas older children (5-9 years) showed greater changes consistent with the immunity debt hypothesis, suggesting different exposure pathways to potential KD triggers. These findings support an age-stratified approach to research focusing on KD pathogenesis, distinguishing infants from older children.
eTable 1. Characteristics of patients with Kawasaki disease diagnosed in 2023–2024 by age group (N=29,841)
eTable 2. Comparison of coronary artery lesion subtypes: before and during versus after the COVID-19 pandemic (N=112,819)
eFigure. Age distribution of patients with Kawasaki disease diagnosed in 2023–2024 (N=29,841)
Data Sharing Statement
References
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
eTable 1. Characteristics of patients with Kawasaki disease diagnosed in 2023–2024 by age group (N=29,841)
eTable 2. Comparison of coronary artery lesion subtypes: before and during versus after the COVID-19 pandemic (N=112,819)
eFigure. Age distribution of patients with Kawasaki disease diagnosed in 2023–2024 (N=29,841)
Data Sharing Statement
