Research led by The Pirbright Institute suggests germ-free (GF) eggs can dramatically increase yellow fever vaccine yields while eliminating the risk of internal microbial contamination.
Writing in the journal Vaccine X, the team demonstrate how GF eggs can produce substantially higher yields of yellow fever vaccine virus than conventional specific pathogen-free (SPF) eggs. They argue their work could potentially improve vaccine production efficiency and helping strengthen global supply.
The study found GF eggs supported up to 1,350 times greater yellow fever virus batch yields than one commercial SPF egg source, and 18 times greater yields than another, while remaining microbiologically sterile.
Yellow fever remains a significant public health threat in parts of Africa and South America, where outbreaks can place considerable pressure on vaccine supplies. The live-attenuated yellow fever vaccine, based on the 17D strain of the virus, is still produced in embryonated chicken eggs using methods that have remained largely unchanged for decades.
Researchers investigated whether GF eggs, produced under strictly sterile conditions, could overcome one of the major challenges facing egg-based vaccine manufacturing: the risk of microbial contamination and inconsistent vaccine yields.
Lead author Professor Munir Iqbal, Head of the Avian Influenza and Newcastle Disease group at The Pirbright Institute, said, “To assess the microbiological status of the eggs, we tested a total of 436 GF eggs using three independent methods, including sterility testing, total viable count analysis and ATP bioluminescence assays.
“Analyses confirmed that the eggs were microbiologically sterile, with only a single contaminated sample detected, which we believe arose from contamination during handling. Our findings have been independently verified by external laboratories and a global vaccine manufacturer, which also confirmed the sterility of the eggs after transportation and incubation.”
The team inoculated GF eggs and SPF eggs from two commercial suppliers with the yellow fever vaccine strain YFV-17D and measured the amount of virus produced.
At 72 hours after infection, GF eggs generated significantly greater viral yields than either SPF source. In addition to producing larger harvest volumes, the eggs supported markedly higher levels of viral replication.
To understand why, researchers examined interferon-beta (IFN-β), a key component of the body's innate antiviral immune response.
Chicken embryo fibroblast cells derived from GF eggs showed significantly lower IFN-β responses following infection than cells derived from SPF eggs. Depending on the time point, IFN-β expression was between 1.6-fold and 5.1-fold lower in cells derived from germ-free embryos.
The researchers believe this reduced antiviral activity may help explain the increased viral replication observed in GF eggs.
"These findings suggest that the absence of microbial exposure may reduce innate antiviral priming, creating conditions that allow the vaccine virus to replicate more efficiently," said Professor Iqbal.
Egg-based vaccine production remains vulnerable to microbial contamination, which can lead to batch failures, production delays and significant financial losses.
Because GF eggs are produced under aseptic conditions and are free from internal microbial contamination, they may offer manufacturers a way to reduce these risks while increasing vaccine output.
This is the first study to evaluate yellow fever vaccine production in germ-free embryonated eggs. Further studies are underway with global vaccine manufacturers to confirm findings at larger scales and assess commercial applicability.
"Our results demonstrate that GF eggs support significantly greater yellow fever vaccine virus replication while remaining microbiologically sterile," said Professor Iqbal. "These findings provide a rationale for further investigation of GF egg technology as a future platform for vaccine production."
Read the paper: Germ-free embryonated eggs support enhanced yellow fever virus 17D vaccine production through reduced interferon-β responses