PHWR Public Health Weekly Report

Syphilis is a chronic, systemic infectious disease caused by Treponema pallidum, a bacterium belonging to the Spirochaetaceae family, and is primarily transmitted between people through sexual contact. Early syphilis is highly infectious and encompasses the primary, secondary, and early latent stages, while late syphilis encompasses late latent and tertiary stages [1]. The average incubation period for syphilis is 21 days (range: 3–90 days), and its primary stage is characterized by the appearance of a painless, indurated ulcer, known as a chancre, at the site where the bacteria entered the body. This lesion resolves in 2 to 8 weeks, even without treatment. Secondary syphilis develops an average of 6 weeks (range: 2–12 weeks) after contact with an infected person or 2 to 8 weeks after the appearance of the chancre, as spirochetes multiply and disseminate systemically, activating the body’s immune response. Systemic symptoms appear along with mucocutaneous lesions, and abnormalities can occur in any organ system; these manifestations spontaneously improve after 3 to 12 weeks. If syphilis is left untreated, there may be a period without clinical symptoms; this does not signify that the disease is not progressing but merely indicates the absence of clinical signs and symptoms. Latent syphilis is defined as a case with a positive specific treponemal antibody test but no clinical symptoms. It is divided into two stages based on the approximate duration of infection; the first year after infection is considered early latent syphilis, a period during which clinical relapse is possible and the patient may be infectious. Ninety percent of relapses occur within the first year, with mucocutaneous relapses being the most common. Late latent syphilis is defined as an asymptomatic infection with unknown duration or that of more than 1 year, representing a non-relapsing and non-infectious period owing to established immunity. Tertiary syphilis manifests 5 to 30 years after the initial infection as cardiovascular syphilis involving the ascending aorta, or neurosyphilis, including meningovascular syphilis, tabes dorsalis, and general paresis; gummas can affect the skin, bones, and liver. Syphilis can also cause in-utero infection, leading to congenital syphilis in the fetus, which, depending on the severity of infection, can result in late-term miscarriage, stillbirth, neonatal death, neonatal infection, or latent infection [2,3].

The high transmission risk of approximately 51–64% [4] facilitates community spread, while congenital syphilis represents a direct threat to maternal and child health through vertical transmission. Furthermore, syphilis is associated with a high burden, because it facilitates the acquisition and transmission of other diseases, increasing infection opportunities, and can progress to severe complications if left untreated [5]. Therefore, it is crucial to recognize syphilis as a public health problem and continuously monitor its occurrence patterns.

In 1954, Republic of Korea (ROK) established a surveillance system with monthly reporting by including “sexually transmitted diseases” in the third category of legally notifiable infectious diseases under the Infectious Disease Prevention Act [6]. Syphilis was under a sentinel surveillance system from 2001 to 2010 and was shifted to a mandatory surveillance system following the reorganization of the legal infectious disease classification system in 2010. It remained under the mandatory surveillance system until 2019, after which it was reverted to a sentinel surveillance system in 2020. While under sentinel surveillance, the need for proactive preparedness by strengthening the surveillance system was raised owing to increasing trends in neighboring countries, leading to a reversion to mandatory surveillance in 2024. In addition, the scope of reportable syphilis was expanded from primary, secondary, and congenital syphilis to include early latent and tertiary syphilis, and individual case-based epidemiological investigations were mandated to understand the overall incidence and risk factors [5].

This study aims to elucidate the characteristics of syphilis incidence in ROK by analyzing its status and epidemiological features under the mandatory surveillance system in 2024.

1. Study Population

A total of 2,790 syphilis cases, classified by stage, that were reported and confirmed from January to December 2024 through the Public Health Information Integrated System, the legal infectious disease reporting system of the Korea Disease Control and Prevention Agency.

2. Data Collection and Analysis

Data on syphilis cases in 2024 were collected from notification and epidemiological investigation forms registered in the Public Health Information Integrated System, while resident registration data for calculating incidence rates were obtained from Statistics Korea. Descriptive analysis was performed on the collected data using Microsoft Excel 2016 (Microsoft), and R version 4.2.1 (The Comprehensive R Archive Network) was used for trend analysis.

1. Incidence Characteristics

In 2024, there were 983, 524, 51, 12, and 1,220 cases of primary syphilis, secondary syphilis, tertiary syphilis, congenital syphilis, and early latent syphilis, respectively, totaling 2,790 cases across the five reportable stages. Early latent syphilis accounted for 43.7% of all syphilis cases in 2024, followed by primary syphilis (35.2%), secondary syphilis (18.8%), tertiary syphilis (1.8%), and congenital syphilis (0.4%) (Table 1).

Table 1 Number and rate^a) of syphilis cases, by stage and demographic characteristics, 2024

Characteristic	Primary syphilis				Secondary syphilis				Tertiary syphilis				Congenital syphilisb)				Early latent syphilis				Overall total
	N	(%)	Rate		N	(%)	Rate		N	(%)	Rate		N	(%)	Rate		N	(%)	Rate		N	(%)	Rate
Sex
Male	841	(85.6)	3.3		444	(84.7)	1.7		36	(70.6)	0.1		4	(33.3)	-		852	(69.8)	3.3		2,177	(78.0)	8.5
Female	142	(14.4)	0.6		80	(15.3)	0.3		15	(29.4)	0.1		8	(66.7)	-		368	(30.2)	1.4		613	(22.0)	2.4
Age group (yr)
≤9	0	(0.0)	-		0	(0.0)	-		0	(0.0)	-		12	(100.0)	5.0		0	(0.0)	-		12	(0.4)	0.4
10–19	36	(3.7)	0.8		22	(4.2)	0.5		0	(0.0)	-		0	(0.0)	-		39	(3.2)	0.8		97	(3.5)	2.1
20–29	334	(34.0)	5.5		172	(32.8)	2.8		4	(7.8)	0.1		0	(0.0)	-		343	(28.1)	5.6		853	(30.6)	14.0
30–39	284	(28.9)	4.3		176	(33.6)	2.7		3	(5.9)	0.0		0	(0.0)	-		320	(26.2)	4.8		783	(28.1)	11.9
40–49	158	(16.1)	2.0		72	(13.7)	0.9		13	(25.5)	0.2		0	(0.0)	-		160	(13.1)	2.0		403	(14.4)	5.2
50–59	109	(11.1)	1.3		51	(9.7)	0.6		12	(23.5)	0.1		0	(0.0)	-		122	(10.0)	1.4		294	(10.5)	3.4
60–69	39	(4.0)	0.5		21	(4.0)	0.3		7	(13.7)	0.1		0	(0.0)	-		118	(9.7)	1.5		185	(6.6)	2.4
≥70	23	(2.3)	0.4		10	(1.9)	0.2		12	(23.5)	0.2		0	(0.0)	-		118	(9.7)	1.8		163	(5.8)	2.5
Regionc)
Capital	569	(57.9)	2.1		334	(63.7)	1.2		29	(56.9)	0.1		3	(25.0)	2.2		695	(57.0)	2.5		1,631	(58.5)	5.9
Chungcheong	120	(12.2)	2.2		42	(8.0)	0.8		1	(2.0)	0.0		3	(25.0)	10.9		119	(9.8)	2.1		284	(10.2)	5.1
Honam	78	(7.9)	1.4		40	(7.6)	0.7		1	(2.0)	0.0		0	(0.0)	-		143	(11.7)	2.5		264	(9.5)	4.7
Gyeongbuk	74	(7.5)	1.5		39	(7.4)	0.8		5	(9.8)	0.1		3	(25.0)	14.7		109	(8.9)	2.2		228	(8.2)	4.7
Gyeongnam	142	(14.4)	1.9		69	(13.2)	0.9		15	(29.4)	0.2		3	(25.0)	9.5		154	(12.6)	2.0		383	(13.7)	5.0
Totald)	983	(35.2)	1.9		524	(18.8)	1.0		51	(1.8)	0.1		12	(0.4)	5.0		1,220	(43.7)	2.4		2,790	(100.0)	5.4

-= Not available. ^a)Per 100,000 population. ^b)Incidence rate per 100,000 births based on provisional number of births in 2024. ^c)Capital: Seoul, Incheon, Gyeonggi, Gangwon; Chungcheong: Daejeon, Sejong, Chungbuk, Chungnam; Honam: Gwangju, Jeonbuk, Jeonnam, Jeju,; Gyeongbuk: Daegu, Gyeongbuk; Gyeongnam: Busan, Ulsan, Gyeongnam. ^d)Percentages by syphilis stage are based on the total number of syphilis cases (2,790)..

The incidence rates per 100,000 population in 2024 by stage were 1.9, 1.0, 0.1, and 2.4 for primary syphilis, secondary syphilis, tertiary syphilis, and early latent syphilis, respectively. Regarding congenital syphilis, all cases were those in newborns, with an incidence rate of 5.0 per 100,000 live births in 2024 (Table 1). The 2020–2023 period was excluded as incidence rates could not be calculated owing to sentinel surveillance. A trend analysis comparing incidence rates from the previous mandatory surveillance period (2016–2019) with those in 2024 showed a significant decreasing trend for primary syphilis (p<0.001), while no significant trends were observed for secondary and congenital syphilis (p=0.624 and p=0.189, respectively) (Table 2) [7]. The year 2024 marked the transition back to mandatory surveillance and the first application of expanded reporting criteria. Because these changes may have led to initial reporting omissions or delays, the data may not fully reflect the actual incidence. Therefore, sustained monitoring and continuous promotion of the new system are essential.

Table 2 Number and rate of primary, secondary, and congenital syphilis cases during the mandatory surveillance period

Category		2016	2017	2018	2019	2024	p for trend testa)
Primary syphilis	No.	1,067	1,454	1,571	1,176	983	<0.001
	Rateb)	2.1	2.8	3.0	2.3	1.9
Secondary syphilis	No.	481	684	680	554	524	0.624
	Rateb)	0.9	1.3	1.3	1.1	1.0
Congenital syphilis	No.	21	10	29	23	12	0.189
	Ratec)	5.2	2.8	8.9	7.6	5.0

^a)Determined by use of Cochrane-Armitage trend test. ^b)Incidence rate per 100,000 population. ^c)Incidence rate per 100,000 births. Data from the article of Korea Disease Control and Prevention Agency (Annual report on the notified infectious diseases in Korea, 2024; 2025) [7]..

In terms of sex, of the 2,790 cases, 2,177 (78.0%) were males and 613 (22.0%) were females. Regarding disease stage, the numbers were as follows: primary syphilis, 841 male (85.6%) and 142 female (14.4%) cases; secondary syphilis, 444 male (84.7%) and 80 female (15.3%) cases; congenital syphilis, 4 male (33.3%) and 8 female (66.7%) cases; tertiary syphilis, 36 male (70.6%) and 15 female (29.4%) cases; and early latent syphilis, 852 male (69.8%) and 368 female (30.2%) cases. The male-to-female case ratio was approximately 8:2 for primary and secondary syphilis and approximately 7:3 for tertiary and early latent syphilis. The incidence rates per 100,000 population were 8.5 and 2.4 for males and females, respectively, making the rate approximately 3.5 times higher in males (Table 1). This finding is similar to that in the United States, where the rate of primary and secondary syphilis in 2023 was reported to be about three times higher in males than in females [8].

Excluding congenital syphilis cases, the age range of the cases was 13 to 91 years. By age group, the highest number of cases occurred in those in their 20s (853 cases, 30.6%), followed by those in their 30s (783, 28.1%), 40s (403, 14.4%), and 50s (294, 10.5%). The incidence rate per 100,000 population was the highest among those in their 20s (14.0), followed by those in their 30s (11.9), 40s (5.2), and 50s (3.4) (Table 1). The age-specific incidence rates of primary and secondary syphilis during the mandatory surveillance periods (2011–2019 and 2024) were consistently the highest among those in their 20s, decreasing with increasing age. Furthermore, two distinct clusters were identified for the periods 2011–2015 and 2016–2019/2024. In 2024, the incidence in the 30s age group was slightly higher than in other years (Figure 1).

Figure 1. Rate of primary and secondary syphilis by age intervals, 2011–2019 and 2024

Regarding the incidence in males by stage and age group, primary syphilis was most common among those in their 20s, and the incidence decreased with age. Secondary and early latent syphilis were most frequent among those in their 30s, with the incidence decreasing with age (Figure 2A).

Figure 2. Number of syphilis cases by sex, stage, and age group: (A) males, (B) females

Regarding the incidence in females, primary, secondary, and early latent syphilis were most common among those in their 20s, and the incidence decreased with age. However, a slight increase was observed in the incidence rate of primary syphilis among those aged 70 years and above and in that of early latent syphilis among those aged 60 years and above (Figure 2B).

Among both males and females, the incidence of syphilis was the highest among sexually active individuals in their 20s and 30s, a characteristic traditionally prominent in cases of sexually transmitted infections (STIs). The unique phenomenon in ROK of higher syphilis incidence among elderly females compared to middle-aged females is presumed to be an artifact of active screening programs. In ROK, syphilis screening is conducted in various situations, such as during hospital or long-term care facility admissions and health check-ups. For cases reported in individuals aged 60 years and above in 2024, the diagnosis was most often made during such screenings. These cases mostly lacked clinical symptoms and risk factors for syphilis and showed low titers on nontreponemal tests.

Based on the reporting date, the monthly average of all syphilis cases was 233, with the highest number of cases (274) occurring in July. For primary syphilis, the monthly average was 82 cases, peaking at 103 cases in July. Regarding secondary syphilis, the monthly average was 44 cases, peaking at 63 cases in October. For early latent syphilis, the monthly average was 102 cases, peaking at 122 cases in both January and August (Figure 3).

Figure 3. Nmuber of syphillis cases by stage and month in 2024

Regarding the distribution of the cases by region, based on the registered address of the cases, the capital area accounted for 1,631 cases (58.5%), with an incidence rate of 5.9 per 100,000 population. The Chungcheong region had 284 cases (10.2%) with an incidence rate of 5.1, while the Honam region had 264 cases (9.5%) with a rate of 4.7. The Gyeongbuk region had 228 cases (8.2%) with an incidence rate of 4.7, and the Gyeongnam region had 383 cases (13.7%) with a rate of 5.0 (Table 1). The capital area accounted for 58.5% of all syphilis cases, as it is the primary residential area for people in their 20s and 30s. Given that incidence rates did not differ significantly from those of other regions, the high number of cases is considered to be due to population size.

2. Epidemiological Characteristics

Regarding clinical symptoms by stage, ulcers were the most common in primary syphilis (549 cases, 62.2%), followed by rash (340, 38.5%), fatigue (46, 5.2%), and fever (33, 3.7%). The characteristic sign of primary syphilis is a hard chancre, often accompanied by non-suppurative, painless regional lymphadenopathy. However, epidemiological investigations revealed that some cases with primary syphilis also reported systemic symptoms such as rash, fatigue, and fever, suggesting the possibility of co-infection with other STIs. In secondary syphilis, rash was the most common symptom (430 cases, 85.5%), followed by ulcer (77, 15.3%), fever (60, 11.9%), and fatigue (57, 11.3%). In addition to these major symptoms, a variety of less frequent symptoms were identified, including lymphadenopathy, chills, myalgia, headache, respiratory and gastrointestinal symptoms, visual abnormalities, and alopecia (Table 3). The diagnoses confirmed in tertiary syphilis cases were neurosyphilis (24 cases), ocular syphilis (8 cases), concurrent neurosyphilis and ocular syphilis (5 cases), and otosyphilis (1 case). The remaining 13 cases were diagnosed as syphilis or syphilis of an unspecified type. The main systemic symptom in tertiary syphilis was fatigue. In cases with neurosyphilis, the common symptoms were headache, paresthesia, cognitive decline, and vision loss. In those with ocular syphilis, vision loss, optic neuritis, retinitis, and eye pain were noted, and in cases with otosyphilis, bilateral tinnitus and hearing loss were confirmed.

Table 3 Major symptoms by stage of syphilis

Major symptomsa)	Primary syphilis	Secondary syphilis	Tertiary syphilis
Ulcer	549 (62.2)	77 (15.3)	2 (5.3)
Rash	340 (38.5)	430 (85.5)	3 (7.9)
Fever	33 (3.7)	60 (11.9)	1 (2.6)
Chills	15 (1.7)	19 (3.8)	0 (0.0)
Muscle pain	26 (2.9)	43 (8.5)	4 (10.5)
Headache	20 (2.3)	25 (5.0)	9 (23.7)
Fatigue	46 (5.2)	57 (11.3)	19 (50.0)
Lymphadenopathy	23 (2.6)	8 (1.6)	0 (0.0)
Other	107 (12.1)	71 (14.1)	33 (86.8)

Unit: n (%). ^a)Multiple responses possible..

The presumed route of infection was domestic for the majority of cases, with foreign-acquired infections accounting for 117 cases (4.2%). Among the 117 foreign-acquired infections, the most common presumed region of infection was the Philippines (22 cases, 18%), followed by Laos (19 cases, 15%), Thailand (14 cases, 11%), Japan (13 cases, 11%), China (9 cases, 7%), and Indonesia (8 cases, 6%).

The World Health Organization recently set a goal to end the syphilis epidemic by reducing its incidence by 90% by 2030 compared to 2020 levels, emphasizing the strengthening of prevention and control policies to achieve this [9]. ROK also established the STI Prevention and Control Plan in 2022 for the systematic management of STIs, including syphilis, setting the elimination of adult (primary and secondary) and congenital syphilis as a mid-to-long-term goal. The 2024 transition to mandatory syphilis surveillance enabled the comprehensive identification of new domestic cases, while epidemiological investigations allowed for the characterization of their demographic and epidemiological features. The main characteristics of new syphilis cases in 2024 were a high incidence among males in their 20s and 30s, a 3.5-fold higher incidence in male cases than that in female cases, and a concentration of cases in the capital area, where a large portion of the younger population resides. These findings are similar to previously known characteristics of syphilis incidence, suggesting that the epidemiological features of the disease have not significantly changed in recent times. Therefore, targeted prevention and screening strategies for males in their 20s and 30s are needed. Furthermore, the fact that early latent syphilis accounts for nearly half of all cases suggests that delays in seeking medical care, diagnosis, and treatment persist, likely owing to concerns about stigma. Consequently, it is necessary to identify the reasons for diagnostic delays and develop strategies to further enhance the accessibility of clinical care and testing.

While it is difficult to identify distinct seasonal patterns, a slight increase in new syphilis cases was observed during the summer months. In particular, the increased occurrence of primary and secondary syphilis in July, stages where symptoms manifest, is likely attributable to better recognition of skin lesions due to lighter clothing and increased frequency of bathing during this period.

An increase in infectious diseases in neighboring countries heightens the potential for domestic importation and subsequent rise in local incidence owing to increased international exchange of people and goods. Recently, an increase in syphilis incidence has been confirmed in neighboring countries such as Japan, Taiwan, and China [5], and ROK also showed a rising trend in cases identified through sentinel surveillance from 2020 to 2023. However, sentinel surveillance data were insufficient for determining the overall scale of incidence or identifying risk factors. In response, syphilis surveillance was converted to a mandatory system in 2024, and epidemiological investigations were implemented to identify sources and routes of infection. As a result, ROK has now determined the 2024 stage-specific case numbers and incidence rates, as well as the scale of incidence by sex, age, and region. Additionally, case-based investigations have made it possible to ascertain clinical symptoms and presumed routes of infection. While there is currently no evidence to suggest that the rising incidence in Japan, Taiwan, and China has affected domestic cases, education and promotion regarding the syphilis situation in these countries, as well as prevention measures for international travelers, should be emphasized.

STIs, including syphilis, are characterized by clandestine transmission through sexual contact. Genital lesions may not be externally visible, and patients often avoid seeking medical care owing to fear of stigma, making such cases difficult to detect through surveillance systems [10,11]. Furthermore, as commercial sex work through traditional red-light districts has declined, new channels such as various new types of entertainment establishments and dating apps have emerged, contributing to a culture where indiscriminate sexual contact has become more prevalent than in the past. Therefore, it is necessary to disseminate accurate knowledge about STIs to the general population, including adolescents and foreign residents. Such education and promotion are crucial for preventing transmission and addressing treatment avoidance among infected individuals. Additionally, effective approaches must be developed to raise awareness about the need for STI screening.

Moreover, in light of cultural shifts such as increased illegal drug smuggling, tattooing, and various cosmetic procedures, it is necessary to develop surveillance and investigation methods for blood-borne outbreaks. The STI-related knowledge and policy comprehension of public health personnel at health centers and other facilities managing STIs must also be enhanced.

By its nature, the clinical management and epidemiological investigation of syphilis inevitably involve intrusion into patients’ private lives. If a patient conceals or provides false information about risk factors owing to fear of moral judgment, it can impede accurate staging and appropriate treatment and obscure the source and route of infection. The prevention and control of syphilis are essential not only for individual health but also for interrupting transmission to others and protecting public health. Therefore, from a public health perspective, autonomous and active participation of individuals is necessary. Furthermore, healthcare and public health institutions must exercise exceptional care in protecting personal information and maintaining confidentiality.

This study, which analyzed the characteristics of syphilis incidence in 2024 following the conversion to mandatory surveillance and the first implementation of epidemiological investigations, is expected to serve as scientific evidence for developing future STI prevention and control policies and operating an effective surveillance system.

Ethics Statement: Not applicable.

Funding Source: None.

Acknowledgments: We acknowledge the staff members responsible for sexually transmitted infection control at public health centers, city and provincial governments, and Korea Regional Center for Disease Control and Prevention for their contributions.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: JHY, EYK, SHH. Data curation: EYK. Formal analysis: EYK. Methodology: EYK. Supervision: JHY. Visualization: EYK, SHH. Writing – original draft: EYK. Writing – review & editing: JHY, SHH.