In Vivo Risk Assessment of Yellow Fever Virus Transmission Through Breastfeeding, and Mechanistic Insights

Yellow fever virus (YFV), a mosquito-borne orthoflavivirus, remains a significant public health concern, especially in regions 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 causing neonatal meningoencephalitis. However, breastfeeding transmission efficiency of the vaccine strains remains unknown, and wild-type strain transmission has been suggested following viral RNA detection in milk. Obtaining direct evidence of breastfeeding-related transmission in humans is challenging as vector-borne exposure confounds analyses, making animal models essential for assessing this risk.

We used an A129 mouse model to investigate YFV transmission via breastfeeding for wild-type and vaccine strains, and human epithelial in vitro models to explore mechanisms of mammary and intestinal barrier crossing.

Wild-type and vaccine strains spread to mammary glands, targeting mainly stromal and immune cells, and are excreted into milk as free and cell-associated virus. In vitro, mammary epithelial cells also support infection, suggesting 2 mechanisms of epithelial crossing. Neonates are susceptible to oral infection, showing higher infection rates for wild-type virus but evidence of neuroinvasion for both strains. These strains infect and cross an in vitro human intestinal barrier model, suggesting this epithelium as a potential viral entry site for neonates. Finally, the virus can be transmitted from infected dams to suckling pups via breastfeeding, though rarely.

This study demonstrates YFV transmission through breastfeeding in an animal model and supports the biological plausibility of this route, highlighting its potential among YFV transmission risks.