CD40LG and viral infectious disease: IgM persistence in serum has been documented following many flaviviral infections, including with West Nile virus for > 7 years, Zika virus for > 2 years, JE virus for up to 1 year, and dengue virus for > 1 year in some subjects in cohort studies.3–9 Similarly, following administration of yellow fever vaccine, a live attenuated flaviviral vaccine, to U.S. residents who had no evidence of infection with yellow fever virus or related flaviviruses before vaccination, almost three-quarters (73%; 29 of 40) of individuals had yellow fever IgM antibodies 3–4 years later.10