Long-term immunity after vaccination against yellow fever in Korean travelers.

While a live-attenuated Yellow fever (YF) vaccine is known to elicit durable immunity, antibody titers may wane after vaccination. This study evaluated the long-term immunity after vaccination against YF among individuals who reside in Korea and received vaccination 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 were vaccinated 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 focus reduction neutralization test (FRNT). Sixty seven subjects were enrolled. The overall positivity rates for FRNT₅₀, IFA and ELISA were 97.0%, 86.6% and 26.9%, respectively, in a time-variable, cross-sectional sample of the cohort. 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 the Korean individuals who were more than five years post-vaccination. IFA testing yielded results similar to FRNT₅₀ testing, which may justify further development of the IFA to screen for waning immunity among those with previous YF vaccination.