Long-Term Immunity after Vaccination against Yellow Fever in Korean Travelers

Although a live-attenuated yellow fever (YF) vaccine is known to elicit durable immunity, antibody titers may wane after vaccination. This study evaluated long-term immunity after vaccination against YF among individuals who resided in Korea and were vaccinated with YF virus- 17D prior to international travel. Serum was collected between December 2018 and December 2019 at the National Medical Center, Republic of Korea, from YF vaccine recipients who had been vaccinated for more than five years prior to sample collection. Long-term immunity against YF was assessed using three serological assays: IgG enzyme-linked immunosorbent assay (ELISA), immunofluorescence assay (IFA), and the focus reduction neutralization test (FRNT). Sixty-seven patients were enrolled in this study. The overall positivity rates for 50% focus reduction neutralization test (FRNT₅₀ ), IFA, and ELISA in a time-variable cross-sectional sample of the cohort were 97.0%, 86.6%, and 26.9%, respectively. While 93.8% (15/16) of samples collected ≥10 years post-vaccination remained positive by FRNT₅₀ , a significant inverse correlation was observed between FRNT₅₀ titer and interval after vaccination (R = - 0.385, P = 0.001). Humoral immunity against YF was well preserved among Korean individuals who were vaccinated more than five years ago. IFA testing yielded results similar to those of FRNT₅₀ testing, which may justify the further development of IFA to screen for waning immunity among those with previous YF vaccination.