From June 2022 until April 2023, the KDCA recommended pre-exposure prophylaxis (PrEP), limited to high-risk groups such as laboratory personnel diagnosing the mpox virus or treating mpox patients.3 This recommendation was guided by the Vaccination Expert Advisory Group. The usage of the vaccine varied based on supply; up to June 2022, 97 individuals were vaccinated with the second-generation smallpox vaccine, CJ-50300 (HK inno.N). After August 2022, the third-generation vaccine, JYNNEOS (Modified Vaccinia Ankara vaccine, Bavarian Nordic), was introduced in the Republic of Korea and used for PrEP.

In April 2023, the first case of community transmission of mpox without international travel history was detected in the Republic of Korea,4 leading to a continuous increase in domestic cases. Similar to overseas epidemiological patterns, the primary group affected domestically was men who have sex with men (MSM), which posed challenges for epidemiological investigations and limited the effectiveness of post-exposure prophylaxis due to the difficulty of identifying close contacts.5 Considering these circumstances, the Republic of Korea expanded the range of high-risk groups to include individuals 18 years and older who may have or are expected to have a risk of infection, such as through sexual contact.

This PrEP program for the expanded high-risk group started in June 2023. This approach follows practices similar to those in the United States, United Kingdom, Australia and Taiwan.678

As shown in the Table 1, the detailed vaccination status by demographic group is outlined. By the end of April 2024, a total of 97 individuals had received the CJ-50300 vaccine, and a total of 6,863 individuals had received their first dose and 3,875 had completed their second dose of the JYNNEOS. CJ-50300 was primarily administered to individuals aged 20–29 (60.8%) and 30–39 (22.7%), with a higher proportion of females (57.7%) than males (42.3%), and exclusively via the subcutaneous route. In contrast, the JYNNEOS was more widely distributed across age groups, with the majority aged 30–39 (46.0% for the first dose, 45.2% for the second dose) and predominantly male recipients (97.6% for the first dose, 97.8% for the second dose). The JYNNEOS was mainly administered intradermally (88.3% for the first dose, 85.0% for the second dose), with a smaller proportion receiving it subcutaneously. To address initial vaccine shortages and with the evidence that the intradermal vaccination was not compromising efficacy or safety,89 an intradermal vaccination method was introduced, which significantly reduced the amount of vaccine used. Subcutaneous vaccination was allowed exceptionally for individuals with a history of keloids.4 To prevent social stigmatization, protect personal privacy, and encourage active participation in vaccination among high-risk groups, information regarding sexual orientation, such as MSM status, was not collected separately.

As of April 2024, 156 cumulative domestic confirmed cases were reported. Fig. 1 shows the mpox epidemiological trends overlapping with the nationwide vaccination status. This figure particularly illustrates the weekly number of mpox confirmed cases alongside the progress of the JYNNEOS vaccination program from March 2023 to March 2024 (Fig. 1). The gray bars represent the weekly confirmed mpox cases, which peaked in April 2023 with 16 cases per week before gradually declining. The red and blue lines depict the number of first and second JYNNEOS vaccine doses administered per week, respectively. Notably, the vaccination campaign gained momentum in May 2023, coinciding with the peak in confirmed cases. The highest number of first doses administered in a single week was 1,304, occurring in mid-May 2023, shortly after the peak in cases.

The evaluation of the mpox vaccine’s effectiveness was not feasible because most confirmed cases received vaccination after their identification, rather than prior. This temporal discrepancy excluded these cases from the effectiveness analysis, resulting in a sample size too small to conduct a statistically meaningful assessment. By the end of April 2024, adverse events were reported exclusively for the JYNNEOS vaccine, totaling 74 cases. All reported events were mild and included symptoms such as fever and localized reactions, with no adverse events reported for the second-generation vaccine.

There are several suggestions for enhancing the response to the mpox vaccination program in the future. Firstly, to reduce the barriers to vaccination, such as stigma and exposure of personal details including sexual orientation, it is crucial to deeply consider these factors when establishing the vaccination strategy.10 Additionally, to effectively assess the vaccination coverage among the high-risk group, securing nationally representative data for the MSM population in the Republic of Korea is essential.11 Furthermore, conducting a vaccine cost-benefit analysis tailored to the Korean population is necessary. For this analysis, it is crucial to examine the burden of mpox disease and the vaccine effectiveness, considering factors such as the relatively mild severity of symptoms,12 transmission primarily spread through close contacts like sexual interactions,13 and the waning vaccine effectiveness.14 Moreover, few studies have addressed the safety profile of the third generation mpox vaccine in the Republic of Korea; only two exist. One is limited to healthcare professionals,15 and the other is a single-center evaluation.4 Consequently, a nationally representative study is vital to comprehensively assess the real-world safety profile of this vaccine.

During an outbreak, the priority is to suppress the epidemic; however, once the epidemic is over, it is crucial to objectively evaluate the benefits and costs of the vaccination. Therefore, the vaccination strategy can be expanded rapidly and effectively based on this risk-benefit analysis to the appropriate target population. Ensuring this preparedness will lead to a robust response in the future.