Many of today’s Yellow Fever Vaccines (YFV) are live-attenuated strains of YF-17D (a virulent African strain). YFV strains with genetic mutations that differentiate them from YF-17D have resurfaced in South America in recent years. A hypothesis is explored that the recent YFV epidemic in Brazil is based at least in part on attenuation of vaccine-conferred immunity against emerging YFV strains.
The serum neutralization potency of YF-17D vaccine recipients against the vaccine strain and virulent African and Chinese strains was excellent. The YFV-EF-504 strain, which emerged in Brazil in 2017–2019, showed no increase in neutralizing titres. New Brazilian strains are attenuated by YF-17D vaccination. In YF-17D vaccine recipients, two amino acid substitutions in YFV surface glycoprotein E reduced serum neutralization. Inhibition of antibody-mediated viral neutralization may be caused by these changes.
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While YFV vaccines have proven effective for decades, these studies suggest that to maintain effectiveness, new vaccines must consider YFV evolution. Until then, visitors to Brazil and other South American countries should use DEET and netting to avoid mosquito bites.