Abstract
Yellow fever virus (YFV) is a mosquito-borne flavivirus that causes over 109,000 severe infections and over 51,000 deaths annually in endemic areas of sub-Saharan Africa and tropical South America. The virus has a transmission cycle involving mosquitoes and humans or non-human primates (NHPs) as the vertebrate hosts. Although yellow fever (YF) is prevented by a live attenuated vaccine (strain 17D), recent epidemics in Angola, the Democratic Republic of the Congo (DRC), and Brazil put great pressure on vaccine stockpiles. This resulted in the World Health Organization (WHO) and Pan American Health Organization (PAHO) implementing, on an emergency basis only, off-label dose-sparing techniques and policies during 2016-2018 to protect as many people in DRC and Brazil as possible from disease during unexpected large outbreaks of YF. Subsequently non-inferiority studies involving full doses compared to fractional doses indicated promising results, leading some policy-makers and scientists to consider utilizing YF vaccine fractional doses in non-emergency scenarios. Although the additional data on the immunogenicity and safety of fractional doses are promising, there are several questions and considerations that remain regarding the use of fractional doses, including differences in the initial antibody kinetics, differences in the immune response in certain populations, and durability of the immune response to fractional doses compared to full doses. Until the remaining knowledge gaps are addressed, full doses instead of fractional doses should continue to be used unless there are insufficient doses of the vaccine available to control outbreaks of YF.