Analysis of the Pertussis Status in Jeju Special Self-Governing Province in 2024

Abstract

Objectives

Pertussis is a sporadic infectious disease, and no large-scale outbreaks have previously occurred in Jeju However, an unprecedented outbreak was anticipated by 2024. This study aimed to analyze the pertussis cases reported in Jeju in 2024 and share the findings.

Methods

We analyzed pertussis cases reported to the Korea Disease Control and Prevention Agency Integrated Disease Management System between January 1 and December 31, 2024. Epidemiological investigation reports were used to assess cases’ scale, location, demographics, and clinical characteristics. Cross-validation was performed using infectious disease reports, laboratory test results, and cluster outbreak reports to determine outbreak size, location, and transmission patterns.

Results

Most cases were reported in Nohyeong-dong, Jeju City (218 cases), and Daecheon-dong, Seogwipo City (13 cases). The number of cases increased in June and peaked in July. Among the 960 reported cases, 832 were students, and males (606 cases) were nearly twice as affected as females (354 cases). The main clinical symptom was coughing, including mild cough (99.6%), whooping cough (17.2%), and paroxysmal cough (29.3%). Although 844 cases (87.9%) had received at least one vaccine dose, only 641 cases (66.8%) had completed the full vaccination series. There were 45 cluster outbreaks in schools, accounting for 678 cases (70.6%).

Conclusions

Cases were concentrated in densely populated areas, predominantly affecting males and school-aged children rather than infants. Cases mainly occurred during the summer, and the sixth-dose vaccination rate was lower than the fifth-dose rate. This study provides a comprehensive analysis of the pertussis outbreak in Jeju and highlights the need for revised response measures to improve pertussis prevention and management.

Key messages

① What is known previously?

Over the past 12 years (2012–2023), only 43 cases were reported in Jeju Special Self-Governing Province.

② What new information is presented?

In 2024, Jeju Special Self-Governing Province reported 960 cases and 45 cluster outbreaks (including secondary outbreaks), with most cases occurring among childhood and adolescence. Additionally, the number of surged mainly during summer, with a higher incidence in males than in females. The sixth vaccination rate was lower than the fifth vaccination rate.

③ What are implications?

This study provides a detailed understanding of the pertussis outbreak in Jeju Special Self-Governing Province. It suggests the need to revise response measures for the proactive prevention and management of pertussis.

Introduction

Pertussis, classified under the “Infectious Disease Control and Prevention Act,” is a representative respiratory disease. pertussis, caused by Bordetella pertussis, has an average incubation period of 7–10 days and spreads through droplets from infected individuals or carriers. The clinical course of pertussis progresses through three distinct stages. During the catarrhal stage, symptoms such as nasal discharge, tearing, and mild coughing appear, making this the most contagious period. The paroxysmal stage is characterized by severe coughing, sometimes accompanied by a distinctive “whooping” sound and, in severe cases, vomiting or difficulty breathing. In the convalescent stage, major symptoms gradually subside [1]. Additionally, pertussis can lead to respiratory complications such as atelectasis and bronchopneumonia. In infants, it may cause severe complications, including pneumonia and cerebral hemorrhage, which can be fatal [1,2].

Following the declaration by the World Health Organization (WHO) of the end of the Public Health Emergency of International Concern (PHEIC) for coronavirus disease 2019 (COVID-19) on May 5, 2023, a global surge in pertussis cases has been observed. According to the Centers for Disease Control and Prevention, the United States has returned to pre-pandemic levels, with over 10,000 cases reported annually. By the 50th week of 2024, the number of cases had increased six-fold compared to the previous year [3]. Similarly, the European Centre for Disease Prevention and Control reported 23,000 cases in 2023. However, within the first quarter of 2024, the number surged to 32,000, with most cases occurring in individuals aged 10–19 years [4]. Republic of Korea (ROK) experienced a comparable increase, with 6,986 pertussis cases reported nationwide as of July 26, 2024. Among these, 6,420 cases (91.9%) occurred in individuals aged 7–19 years [5].

Pertussis vaccination is designated as a national immunization under the “Infectious Disease Control and Prevention Act,” and ROK provides financial support for vaccination through the “National Childhood Immunization Program.” The available pertussis vaccines include DTaP, Tdap, and Td. Generally, infants receive three doses of the DTaP vaccine at two-month intervals, followed by additional doses at 15–18 months and 4–6 years of age. An additional dose of Tdap or Td is administered at 11–12 years of age to complete the six-dose vaccination schedule. For adults, a Tdap or Td booster is recommended every ten years, and pregnant women are advised to receive the Tdap vaccine between 27 and 36 weeks of gestation to confer immunity against fetal pertussis [1]¹⁾.

ROK promotes timely vaccination through the Standard Immunization Schedule [1]. In 2023, the national childhood immunization coverage for the DTaP vaccine was relatively high, with rates of 97.2% at 1 year, 95.6% at 3 years, and 93.9% at 6 years [6]. Additionally, a 2024 vaccination status survey conducted by the Korea Disease Control and Prevention Agency (KDCA) and the Ministry of Education found that the immunization completion rates for elementary and middle school entrants were 92.3% and 76.4%, respectively, indicating lower coverage among middle school entrants but an overall high level of immunization [7]. However, despite the high vaccination coverage, ROK experienced a significant pertussis outbreak in 2024, with an exceptionally high incidence in the Jeju Special Self-Governing Province among the 17 nationwide regions.

This study aimed to examine the epidemiological characteristics of pertussis cases in Jeju in 2024 by conducting an in-depth analysis of cases in a localized setting to provide insights for more effective disease control strategies.

Methods

When a healthcare institution suspects an infectious disease, it submits an infectious disease occurrence report. In Jeju Special Self-Governing Province, six public health centers (three in Jeju City and three in Seogwipo City) received these reports through the KDCA Integrated Disease Management System (eid.kdca.go.kr) and subsequently reported them to the KDCA. Additionally, healthcare institutions send pathogen samples to designated testing laboratories, which then issue laboratory test results. Based on infectious disease occurrence reports and laboratory test results, local public health centers (at the city, county, or provincial level) conduct epidemiological investigations of suspected cases and compile epidemiological investigation reports. If an outbreak is identified, defined as two or more pertussis cases (with at least one confirmed case) that have epidemiological links in terms of time and location [1], a cluster outbreak report is prepared.

This study analyzed epidemiological investigation reports submitted to the KDCA in 2024 to assess the scale, geographic distribution, and demographic and clinical characteristics of cases in the Jeju Special Self-Governing Province. Cross-validation was conducted using infectious disease occurrence reports and laboratory test results. Outbreak patterns, including cluster size, location, and transmission routes, were examined using cluster outbreak reports.

Data sources included the KDCA Infectious Disease Portal (https://dportal.kdca.go.kr/) for national pertussis case trends (2012–2024), the Statistics Korea National Statistics Portal (kosis.kr) for Jeju’s 2024 resident population, and the Jeju Special Self-Governing Provincial Office of Education (www.jje.go.kr) for school-related data. Descriptive statistical analyses were performed using Microsoft Office Excel 2021.

Results

Over the past 12 years (2012–2023), Jeju Special Self-Governing Province recorded 43 cases. However, following the first reported case in March 2024, a total of 960 cases (including one reinfection) were recorded throughout the year. As of March 5, 2025, the nationwide incidence rate of pertussis in ROK, based on the KDCA’s Infectious Disease Surveillance System, was 93.4 cases per 100,000 people, whereas Jeju reported an incidence rate of 142.2 cases per 100,000 population. Among the 17 nationwide regions in ROK, Jeju recorded the second-highest incidence rate, closely following Incheon (143.5 cases per 100,000 population).

1. Regional Distribution

Jeju Special Self-Governing Province consists of two administrative cities, Jeju City and Seogwipo City, comprising 43 administrative districts (26 in Jeju City and 17 in Seogwipo City). Among the 960 cases reported in Jeju in 2024, 867 occurred in Jeju and 93 in Seogwipo. The highest number of cases in Jeju was reported in Nohyeong-dong (218), Yeondong (111), and Ido 2-dong (81). In Seogwipo, the highest numbers were recorded in Daecheon-dong (13), Donghong-dong (13), and Daejeong-eup (11) (Figure 1A). The incidence rate per 100,000 population by administrative district was highest in Jeju City’s Dodu-dong (490.5), followed by Nohyeong-dong (397.1) and Oedo-dong (312.5). In Seogwipo City, the highest rates were recorded in Cheonji-dong (265.2), Daecheon-dong (94.8), and Seohong-dong (80.9) (Figure 1B).

Figure 1. Regional status of pertusscis cases in Jeju Special Self-Governing Province (2024).

(A) Pertussis cases distribution by region in Jeju Special Self-Governing Province (2024). (B) Pertussis cases distribution by region in Jeju Special Self-Governing Province (per 100,000 population) (2024).

2. Seasonal Factors

A seasonal pattern was observed in cases reported in Jeju. The first case was reported in epidemiological week 11, and the number of confirmed cases began to increase by week 25 (June). A peak was observed in week 27, with 126 cases. Although another increase was noted in week 29, the number of cases began to decline from week 30 onward. After the peak, the weekly case numbers fluctuated but remained below 30 cases per week (Figure 2).

Figure 2. The status of cluster outbreaks and pertussis cases in Jeju Special Self-Governing Province (2024).

3. Demographic Factors

Among the 960 cases reported in Jeju, 832 (86.7%) occurred in individuals aged 18 years or younger. By age group, 20 (2.1%) were reported in infants and toddlers (0–5 years), 380 (39.6%) in childhood (6–12 years), and 432 (45.0%) in adolescence (13–18 years). Regarding sex distribution, 606 (63.1%) were male, and 354 (36.9%) were female, indicating that male were approximately twice as frequent as female (Table 1).

Table 1. The status of pertussis cases in Jeju Special Self-Governing Province (2024) (as of December 31, 2024).

Category	Jeju Special Self-Governing Province			Proportion (%)
	Population (n)	Case (n)	Per 100,000 population (n)
Total	670,368	960	143	100.0
Sex
Male	335,255	606	180	63.1
Female	335,113	354	105	36.9
Life cycle
Infants and toddlers (0–5 years of age)	23,121	20	86	2.1
Childhood (6–12 years of age)	46,350	380	819	39.6
Adolescence (13–18 years of age)	42,536	432	1,015	45.0
Young adulthood (19–29 years of age)	76,020	14	18	1.4
Adulthood (30–64 years of age)	355,356	93	26	9.7
Old Age (65 years of age or older)	126,985	21	16	2.2

4. Clinical Information

Among the 960 cases reported, 958 exhibited symptoms, with a mild cough being the most common. Paroxysmal cough was observed in 281 cases, while the characteristic "whooping" cough was present in 165 cases. And difficulty breathing was experienced in 9 cases. Regarding medical care utilization, 920 cases (95.8%) were treated as outpatients, 12 (1.3%) visited the emergency room, and 28 (2.9%) required hospitalization. No deaths were reported (Table 2).

Table 2. Main clinical symptoms of pertussis cases in Jeju Special Self-Governing Province (2024).

Category		Case (n)	Proportion (%)	Detail
Total		960	100.0	-
Presence of symptoms	Asymptomatic	2	0.2	-
	Symptomatic	958	99.8	If a cases has multiple symptoms, they are recorded redundantly
	Mild cough	956	99.6
	Whooping cough	165	17.2
	Paroxysmal sough	281	29.3
	Difficulty breathing	9	0.9
	Other symptoms	57	5.9	Runny nose (16), fever (20), sore throat (18), chest pain (3)
Healthcare utilization	Outpatient care	920	95.8	-
	Emergency room	12	1.3	-
	Hospitalization	28	2.9	-
Survival status	Survival	960	100.0	-
	Death	0	0.0	-

5. Vaccination Status

Among the 960 pertussis cases, 844 (including 16 cases with partial vaccination records) had received at least one vaccine dose, accounting for 87.9% of the cases. However, only 641 cases (66.8%) had completed all recommended vaccinations on schedule. While cases due for the third or fourth dose received their vaccinations on time, among the 812 cases eligible for the fifth or sixth dose, only 628 received them as scheduled. Among cases with incomplete vaccinations, doses 1–4 were administered on schedule; however, delays were observed starting from the fifth dose. 19 cases (2.0%) had received fewer than five doses despite being eligible, while 168 cases (17.5%) had received fewer than six doses despite being eligible (Table 3). The median interval between the last vaccination and pertussis diagnosis was 1,687 days, with a mean of 1,746 days (standard deviation: 1,063 days). The shortest interval was 0 days, whereas the longest was 7,934 days. By age group, the mean interval was 1,645 days (approximately 4.5 years) for individuals aged 1–10 years and 1,701 days (approximately 4.6 years) for those aged 11–19 years. Among 14-year-olds, who had the highest number of cases, the mean interval was 1,209 days (approximately 3.3 years).

Table 3. The status of pertussis vaccinations in Jeju Special Self-Governing Province (2024).

No. of vaccinations		Case (n)	Vaccination rate by case age group (%)	Complete vaccination ratea) (%)	Detail
Total		960	100.0	100.0	-
1st		-	100.0	97.5	-
2nd		-	100.0	97.5	-
3rd	3 doses	2	100.0	97.5	-
	<3 dosesb)	-			Eligible for 3rd but received fewer than 3 doses
4th	4 doses	14	100.0	96.1	-
	<4 dosesb)	-			Eligible for 4th but received fewer than 4 doses
5th	5 doses	299	94.0	94.4	-
	<5 dosesb)	19			Eligible for 5th but received fewer than 5 doses
6th	6 doses	326	66.0	-	-
	<6 dosesb)	168			Eligible for 6th but received fewer than 6 doses
The others		16	-	-	No history
		116	-	-	Partial history

^a)DTaP vaccination rates for 1-year-olds, 2-year-olds, and 6-year-olds residing in Jeju Special Self-Governing Province [6]. ^b)Calculated based on the maximum point of each vaccination schedule (3rd dose at 1 year old, 4th dose at 2 years old, 5th dose at 6 years old, and 6th dose at 12 years old).

6. Cluster Outbreaks

The 45 cluster outbreaks (including secondary outbreaks) were reported in Jeju, involving 678 cases (70.6%). The average number of cases per outbreak was 15, with a minimum of 2 and a maximum of 61 cases. A total of 37,531 individuals were exposed to outbreak settings, with an average of 834 common exposure contacts per outbreak. All cases occurred in educational institutions, including kindergartens, elementary schools, middle schools, and high schools. However, 85 schools (including kindergartens) additionally reported sporadic pertussis cases that were not classified as outbreak sites. The locations of reported outbreaks included 2 kindergartens, 17 elementary schools, 9 middle schools, and 11 high schools. The number of cases reported from these settings was 5 in kindergartens, 254 in elementary schools, 229 in middle schools, and 190 in high schools (including one reinfection case) (Figure 2).

Index cases and symptomatic contacts were traced to investigate outbreak-related transmission patterns. The first outbreak occurred at Middle School A (School A), where a single case was confirmed in one class. Subsequent cases emerged within the same class before spreading to other classes and grades. The intrafamilial transmission was frequently observed, particularly among siblings attending different schools. However, no clear epidemiological link was identified between family transmission and outbreaks in other schools.

Discussion

This analysis identified various characteristics of cases expected to occur in Jeju Special Self-Governing Province in 2024. One key factor contributing to the high incidence of pertussis in Jeju is the concentrated population in Jeju City, with particularly high population densities observed in specific administrative districts, including Nohyeong-dong (397.1 per 100,000 population), Oedo-dong (312.5 per 100,000 population), and Yeon-dong (260.4 per 100,000 population). However, although Dodu-dong recorded the highest incidence rate (490.5 per 100,000 population), it includes Jeju International Airport and has a relatively low population density compared to other regions. Most cases in this area were linked to two local elementary schools, with eight cases reported at each school. Therefore, excluding Dodu-dong, areas such as Nohyeong-dong, Oedo-dong, and Yeon-dong—collectively referred to as the “downtown” of Jeju City—exhibited the highest incidence. These areas are characterized by concentrated urban infrastructure and residential apartment complexes, which contribute to higher population density and, consequently, a greater number of cases.

Although pertussis is generally considered a non-seasonal infectious disease, a sharp increase in the number of cases was observed during the summer months. The trend of rising cases in June peaking in July aligns with national and international findings. Similar seasonal patterns were observed during pertussis outbreaks in ROK in 2015, 2018, and 2024, with cases increasing in June, peaking in July, and subsequently declining. In June 2015, 19 cases were reported, peaking at 58 in July, followed by a decline in August. Similarly, in June 2018, 67 cases were reported, peaking at 169 cases in July before decreasing. This seasonal pattern was also reported in Europe before the WHO declared COVID-19 a PHEIC on March 11, 2020. Between 2014 and 2018, seasonal pertussis outbreaks were reported in Europe, predominantly during the summer months [8]. In the United States, an analysis of 209,100 pertussis cases reported over an eight-year period found that cases peaked in July and August [9].

A notable sex disparity was observed. While European data from 2018 [8] indicated a higher incidence of pertussis in females than in males (male-to-female ratio, 0.8:1), the trend in Jeju was reversed. In Jeju, the male-to-female ratio of pertussis cases was 1:0.58, indicating that the incidence in males was nearly twice that in females. This pattern was consistent with nationwide data, where 27,931 (58.3%) of the total cases were male and 19,957 (41.7%) were female. However, no definitive explanation has been identified for the higher incidence in males.

Additionally, school-based cluster outbreaks were the primary driver of the surge in pertussis cases in Jeju. Among 812 cases in childhood and adolescence, 82.9% (673) were linked to school outbreaks. The transmission pathways in these outbreaks were likely facilitated by densely populated classroom environments, which allowed for easier disease spread. Furthermore, transmission is suspected to have extended to other schools through contact with private educational institutions (e.g., hagwons). However, international studies have reported higher incidence rates among infants under one year of age than among children and adolescents [8,10]. This discrepancy suggests that further comparative studies with nationwide and regional pertussis trends are needed to determine whether the high incidence of pertussis in children and adolescents represents a broader trend in ROK.

The proportion of pertussis cases among children under five years of age in Jeju was relatively low, accounting for only 2.1% (20 cases) of the total cases. In contrast, the incidence was significantly higher in childhood and adolescence (6–18 years old). This pattern is likely attributable to lower sixth-dose vaccination coverage. In 2024, the sixth-dose vaccination rate among cases in Jeju was projected to be 66.0%, compared to 94.0% for the fifth dose. Immunity conferred by pertussis vaccination wanes over time, increasing susceptibility to infection. Vaccine-induced immunity typically lasts between 4 and 12 years, with a marked increase in pertussis positivity rates beginning four years after the last dose [11,12]. A 2012 study on age-specific national immunization antibody levels in ROK [13] reported a decline in antibody titers after 11 years of age, with geometric mean titers of 64.78 EU/ml in individuals under 10 years, 46.91 EU/ml in those aged 11–20 years, 40.34 EU/ml in those aged 21–30 years, 38.52 EU/ml in those aged 31–40 years, 42.57 EU/ml in those aged 41–50 years, 44.65 EU/ml in those aged 51-60 years, and 43.59 EU/ml in 61 and older, even lower levels in older age groups. These findings suggest that the 2024 pertussis outbreak in Jeju primarily affected upper elementary, middle, and high school students rather than younger children in kindergartens or daycare centers.

Another potential contributing factor to the rise in pertussis cases is the increasing prevalence of pertactin-deficient (PRN^–) B. pertussis strains [14-16], which may reduce the efficacy of the acellular pertussis (aP) vaccine used in ROK. Acellular vaccines, which contain purified pertussis antigens (pertussis toxin [PT], filamentous hemagglutinin [FHA], pertactin [PRN], etc.), are associated with lower reactogenicity and fewer adverse effects than whole-cell pertussis vaccines. However, acellular vaccines provide immunity for a shorter duration and have lower infection prevention efficacy. The loss of PRN in circulating B. pertussis strains may create selective pressure favoring vaccine escape mutants, leading to an increased incidence of pertussis [15,17]. Recent studies [18] support this hypothesis, suggesting that although acellular vaccines reduce disease severity, they may be less effective in eliminating B. pertussis colonization. Three-component DTaP vaccines, which contain PT, a major exotoxin secreted by B. pertussis; FHA, a host cell adhesion protein; and PRN, an outer membrane protein, provide better protection than two-component DTaP vaccines, which include only PT and FHA. Therefore, maintaining PRN-containing DTaP vaccination is recommended, and further research is needed to assess its long-term effectiveness.

Concerns have also been raised regarding improved diagnostic capabilities and the potential for misdiagnosis, both of which contribute to an increased pertussis case count. In developing countries, diagnostic testing limitations often result in underreporting of infectious diseases [19]. In contrast, ROK has systematically enhanced its diagnostic capacity by implementing annual quality assessment programs for infectious disease testing [20], allowing for more accurate case detection. Therefore, improved diagnostics may have contributed to the higher number of reported cases. However, misclassification and misreporting may have also affected the case count. During the early stages of the outbreak, some cases of Bordetella parapertussis infection were mistakenly reported as pertussis due to unfamiliarity with the diagnostic criteria [1]. The KDCA later provided direct guidance to each responsible officer to ensure accurate reporting.

Furthermore, cases of Bordetella holmesii, a species closely related to B. pertussis, were included in the pertussis case counts, contributing to overestimation. The KDCA identified B. holmesii in 60 of the 234 samples initially reported as B. pertussis by private laboratories [5]. B. holmesii presents with symptoms similar to those of pertussis and shares the IS481 gene, which is commonly targeted in pertussis PCR assays. Many clinical laboratories use PCR assays that detect IS481 but do not differentiate between B. pertussis and B. holmesii. Consequently, cases diagnosed using these assays may be misclassified as pertussis. The KDCA implemented additional toxin gene (ptx) testing to distinguish between these two pathogens. However, the current reporting system classifies cases as pertussis even when toxin genes are not detected, raising concerns regarding overestimation [5].

This study has a few limitations. First, although the sixth-dose vaccination rate among cases is known, population-wide vaccination coverage data for Jeju are unavailable, making it difficult to evaluate vaccine effectiveness. Second, epidemiological investigations have focused on school settings, limiting insights into transmission via external activities, such as private academies. Third, data on PRN^– strains were unavailable, preventing an assessment of their role in the outbreak.

This analysis highlighted that the cases were concentrated in densely populated residential districts, with school outbreaks serving as the primary transmission drivers. Males were disproportionately affected, and cases were more frequent among childhood and adolescence than among infants and toddlers. A seasonal pattern of increasing cases from June to July was confirmed. A lower sixth-dose vaccination coverage was also observed. This study provides a detailed understanding of the pertussis outbreak in Jeju Special Self-Governing Province and suggests the need to revise response measures for the proactive prevention and management of pertussis.