Assessing yellow fever outbreak potential and implications for vaccine strategy

Area of Science:

• Epidemiology
• Mathematical Modeling
• Public Health

Background:

• Yellow fever (YF) is a vector-borne viral hemorrhagic fever endemic in Africa and South America.
• Despite vaccination programs, significant YF outbreaks persist, highlighting data gaps in infection rates and seroprevalence.
• Mathematical modeling is essential for estimating YF burden, outbreak risk, and informing vaccination strategies.

Purpose of the Study:

• To develop a dynamic, stochastic model for YF transmission.
• To estimate the force of infection from sylvatic reservoirs and the basic reproduction number for human-to-human transmission.
• To evaluate the World Health Organization's (WHO) EYE Strategy vaccination coverage targets (50%, 60%, 80%) against effective reproduction number (R) thresholds.

Main Methods:

• A dynamic, stochastic YF transmission model incorporating environmental covariates.
• Estimation of force of infection from sylvatic (non-human primate) spillover.
• Calculation of the basic reproduction number for human-to-human transmission.
• Analysis of WHO EYE Strategy vaccination coverage targets (50-80%) against R thresholds.

Main Results:

• The 2022 EYE Strategy vaccination coverage is sufficient to reduce the average annual effective reproduction number (R) below 1 in most African regions, contingent on vaccination effectiveness.
• Current coverage is insufficient to reduce R below 0.5, which is necessary to eliminate outbreaks in regions with significant seasonal transmission variations.
• Achieving the 2026 EYE Strategy targets significantly reduces the proportion of regions where R exceeds 0.5.

Conclusions:

• Current YF vaccination strategies, while reducing transmission, may not be sufficient to eliminate outbreaks entirely, particularly in areas with high seasonal transmission.
• Future vaccination targets, such as those for 2026, show promise in further mitigating YF transmission risks.
• Robust mathematical modeling is vital for assessing the impact of vaccination campaigns and guiding public health policy for YF control.