JYNNEOS vaccine safety monitoring in the Republic of Korea, 2022: a cross-sectional study.

Background: With the recent global mpox outbreak, the JYNNEOS vaccine (Modified Vaccinia Ankara-Bavarian Nordic) was developed as a third-generation smallpox vaccine and initially favored for mpox immunization. Vaccine-associated side effects contribute to vaccine hesitancy. Consequently, tracking adverse events post-immunization is crucial for safety management. This study used data from the national active vaccine safety surveillance conducted in Korea from August 25 to November 24, 2022 to detect potential safety signals and adverse events.

Methods: Data on health conditions following vaccination were gathered from web-based surveys and reported via active surveillance through the Immunization Registry Information System. This follow-up system functioned via a text message link, surveying adverse events and health conditions beginning on the second day post-vaccination. Information about specific adverse events, including both local and systemic reactions, was collected.

Results: The study included 86 healthcare workers who had received at least 1 dose of the JYNNEOS vaccine. Among the respondents, 79.1% reported experiencing at least 1 adverse event, with the majority being local reactions at the injection site. The incidence of adverse events was higher following the first dose (67.9%) than after the second dose (34.4%). The most frequently reported adverse event for both doses was mild pain at the injection site.

Conclusion: The study provides crucial information on the safety of the JYNNEOS vaccine, demonstrating that most adverse events were manageable and predominantly localized to the injection site. Nonetheless, additional research is needed on the safety of various vaccine administration techniques and the vaccine's effects on broader demographics.