Yellow fever is an acute hemorrhagic disease caused by a Flavivirus, with fatality rates exceeding 30 % in severe outbreaks. Theiler repeatedly passaged the virus in chick embryos, attenuating its virulence and generating the “17D strain.” This strain triggers strong protective immunity in humans while remaining highly safe. When mass vaccination started in 1937, deaths in endemic regions dropped dramatically. Even today, travelers to certain areas must show a yellow-fever vaccination certificate (“yellow card”), and Theiler’s work is embedded in the International Health Regulations. His achievement is a landmark in vaccine history and served as a template for later live-attenuated vaccines against polio, measles, and other diseases.

Before Theiler, yellow-fever vaccines derived from mouse brains produced severe adverse reactions, notably post-vaccinal encephalitis. Theiler serially passaged the virus in a non-human host (chick embryos), altering receptor affinity and replication kinetics so that virulence was lost while antigenic epitopes remained intact. Subsequent molecular studies showed that the 17D genome contains mutations in NS2A, NS4B, and other regions, diminishing its capacity to evade interferon responses. Serologically, a single dose elicits neutralizing antibody titers (PRNT50 ≥ 10³) that persist for decades. Limited, controlled replication of the 17D virus in vivo also induces robust T-cell-mediated immunity. Theiler thus established the paradigm of “host-adaptation attenuation,” a textbook protocol for RNA-virus vaccine development.

Theiler serially passaged the Asibi wild-type strain 176 times in SPF chick embryos, fixing >20 nonsynonymous mutations, including substitutions at E protein aa52 and aa173. Both 17D-204 and 17DD sublines exhibit markedly reduced neuro- and viscerotropism, with LD₅₀ values elevated by ~3 orders of magnitude in marmosets and humans. Comparative genomics suggests that the NS4B Phe→Ser swap attenuates IFN-α/β signaling antagonism, while deletion of an E-HIS fragile motif limits vascular endothelial permeability. Immunologically, the vaccine induces a balanced Th1/Th2 cytokine profile and promotes long-lived memory B-cell formation. Reverse-genetics systems based on full-length cDNA clones now exploit the 17D backbone as a recombinant RNA-vaccine platform for multivalent constructs against dengue, Zika, Lassa, and others. Theiler’s work empirically optimized the trade-off between host-adaptation evolution and antigenic integrity, a cornerstone in contemporary Flavivirus vaccinology.