In vivo risk assessment of yellow fever virus transmission through breastfeeding, and mechanistic insights

Yellow fever virus (YFV), a mosquito-borne flavivirus, remains a significant public health threat, especially in areas with low vaccine coverage. Since 2010, yellow fever vaccination is not recommended for breastfeeding women due to reported cases of vaccine strain transmission through breast milk, leading to neonatal meningoencephalitis. However, the efficiency of YFV vaccine strain transmission via breastfeeding remains unknown, and wild-type strains transmission may be suspected based on viral RNA detection in breast milk. Direct evidence of breastfeeding-related transmission in humans is challenging to obtain given the confounding presence of vector-borne transmission, making animal models crucial for evaluating this risk. In this study, the A129 mouse model was used to investigate YFV transmission via breastfeeding for both wild-type and vaccine strains. Results show that both strains can spread to mammary glands, leading to viral detection in breast milk as free viral particles and cell-associated virus, with similar viral loads for all strains. Mammary stromal and immune cells are primary targets of YFV in vivo, while mammary epithelial cells also support infection, suggesting two possible mechanisms of mammary epithelial crossing. Neonates are found to be susceptible to oral infection, with higher infection rates for the wild-type strain but evidence of neuroinvasion for both strains. Both strains can infect and cross an in vitro human intestinal barrier model, indicating this epithelium as a potential viral entry site for neonates. Finally, this study confirms the existence of YFV transmission through breastfeeding in an animal model, highlighting the need to consider it among transmission risks.