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Yellow Fever Outbreaks and Reactive Vaccination in Africa, 2010-2025: A Scoping Review and Analysis

This scoping review and analysis of Yellow Fever outbreaks in Africa from 2010 to 2025 reveals that while the time to initiate reactive vaccination campaigns decreased after the 2017 EYE strategy was implemented, high morbidity and mortality persist, indicating that more timely responses are still necessary to achieve effective control.

Original authors: Carlos Haring, Sayana Lee, David Dowdy, Seth Judson

Published 2026-07-31

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CC BY 4.0

Original authors: Carlos Haring, Sayana Lee, David Dowdy, Seth Judson

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the world of public health as a giant, bustling city where invisible invaders—viruses—sometimes try to break in. One of the sneakiest of these invaders is the Yellow Fever virus, a mosquito-borne troublemaker that can turn a healthy person very sick very quickly. To stop it, the city has a special emergency plan: a "reactive vaccination campaign." Think of this like a fire department rushing to a blaze. The moment a fire (an outbreak) is spotted, the firefighters (health teams) race to the scene to spray water (vaccines) on the surrounding houses to stop the fire from spreading. But here's the catch: if the fire trucks are stuck in traffic or take too long to get the water ready, the fire can grow huge before they even arrive. Scientists and doctors have been watching this race closely, wondering if the new emergency protocols put in place recently have actually made the fire trucks faster and the water coverage better. This is the story of that race: did the new plan help the city catch the fire sooner, or are the trucks still stuck in the same old traffic jams?

This paper is a "scoping review," which is basically a giant detective story where the authors, Carlos Haring and his team, gathered every report they could find about Yellow Fever outbreaks in Africa between 2010 and 2025. They wanted to see if the "Eliminate Yellow Fever Epidemics" (EYE) strategy, which started in 2017, actually changed the game. Specifically, they looked at two main things: how long it took for the "fire trucks" (vaccination teams) to arrive after the first "smoke" (the first sick person) was seen, and how big and deadly the outbreaks were.

The authors found that the race is still a bit of a slog. On average, it took about 90.5 days from the moment the first person got sick to the moment the vaccination campaign started. That's roughly three months of waiting while the virus spreads. When they split the timeline in half, comparing the years before the new strategy (2010–2016) to the years after (2017–2025), they saw a tiny improvement. Before the new plan, the average wait was 108 days; after, it dropped to 79 days. However, the authors are careful to say this isn't a huge victory. Statistically, this drop isn't significant enough to say the new plan definitely fixed the traffic jam. In fact, some of the most recent outbreaks still waited over 100 days for help. The fastest response was a lucky 17 days in Ghana in 2021, but the slowest was a painful 192 days in Kenya in 2022.

The size of the outbreaks didn't really change either. Whether it was before or after the new strategy, the median number of sick people per outbreak was around 90 to 94. The "Case Fatality Ratio" (the percentage of sick people who died) did look a bit better in the later years, dropping from 8.5% to 4.9%, but the authors suggest this might just mean doctors are finding more mild cases now rather than fewer people dying. They also noted that while the speed of vaccination per day went up slightly, the difference wasn't statistically significant.

So, what's the verdict? The paper suggests that while the new strategy might have helped a little bit, the "fire trucks" are still taking too long to get to the fire. The authors argue that we can't just say the problem is solved. They point out that outbreaks are popping up in new places, like East Africa, which weren't always considered high-risk zones. The main takeaway is that to truly stop these outbreaks, we need to keep working on getting vaccines to people faster, finding more cases earlier, and maybe even inventing new medicines to treat the sick, because waiting three months for a vaccine is still too long when lives are on the line.

Technical Summary: Yellow Fever Outbreaks and Reactive Vaccination in Africa, 2010–2025

Problem Statement
The control of Yellow Fever (YF) in Africa relies heavily on the rapid containment of outbreaks through reactive vaccination campaigns (RVCs), a core objective of the 2017–2026 Eliminate Yellow Fever Epidemics (EYE) strategy. While the EYE strategy aims to protect at-risk populations and prevent international spread, the transition of this strategy in 2026 necessitates an evaluation of its impact. Specifically, it remains unclear how the implementation of the EYE strategy has altered the epidemiology of YF outbreaks and the operational efficiency of response mechanisms. Prior modeling suggests that vaccination delays increase outbreak size, but empirical data comparing outbreak patterns and response times pre- and post-EYE implementation in Africa has been lacking.

Methodology
The authors conducted a scoping review and quantitative analysis of YF outbreaks in Africa from 2010 to 2025. The study utilized both peer-reviewed literature and grey literature, adhering to PRISMA-ScR guidelines.

Data Selection: Outbreaks were defined as events with at least one confirmed YF case per WHO standards. To prevent double-counting, multiple reported events triggering a single RVC were grouped as one outbreak.
Periodization: The analysis compared two distinct periods: pre-EYE (2010–2016) and post-EYE (2017–2025), using February 14, 2017 (the declaration of the end of the Angola/DRC outbreak) as the cutoff.
Metrics: Key indicators included the time from the index case to the initiation of RVCs, RVC coverage (proportion of the target population vaccinated), daily vaccination rates, outbreak size (reported cases), and Case Fatality Ratios (CFRs).
Statistical Analysis: Descriptive statistics were calculated using R. The Wilcoxon rank sum test compared non-normally distributed continuous variables (time to RVC, case counts), while Welch's t-test was used for pooled estimates of vaccination rates and CFRs. Outbreaks with uncertain epidemiological links or those affected by unusual delays during the COVID-19 pandemic were excluded from specific time-to-RVC analyses.

Key Contributions
This study provides the first comparative quantitative assessment of YF outbreak characteristics and reactive vaccination performance in Africa across the pre- and post-EYE strategy eras. It offers a granular dataset of 43 outbreaks, with detailed analysis on 20 outbreaks where index case and RVC dates were confirmed, and 29 outbreaks with complete vaccination coverage data. The work highlights the current state of YF control as the global community approaches the 2026 transition of the EYE strategy.

Results

Time to Reactive Vaccination: The overall median time from the index case to RVC was 90.5 days (IQR: 53.8–113.2). While the median time decreased from 108 days (2010–2016) to 79 days (2017–2025), this reduction was not statistically significant ( $p = 0.55$ ). Significant variability persisted, with delays ranging from 17 days (Ghana, 2021) to 192 days (Kenya, 2022). Notably, five of the eight most recent outbreaks experienced delays exceeding 100 days.
Vaccination Coverage and Rates: The overall pooled vaccination rate was 7.9% of the target population per day. Post-2016, the rate increased from 6.1% to 9.8% per day, though this was not statistically significant ( $p = 0.27$ ). Median RVC coverage was 94% (IQR: 88%–102%), with some estimates exceeding 100%, potentially indicating inaccuracies in population estimates or intra-region mobility.
Morbidity and Mortality: The median outbreak size was 91 cases (IQR: 16–276) for 2010–2016 and 94 cases (IQR: 49–377) for 2017–2025, a difference that was not statistically significant ( $p = 0.77$ ). The overall CFR was 6.6%. There was a reduction in CFR from 8.5% (2010–2016) to 4.9% (2017–2025), but this change was also not statistically significant ( $p = 0.32$ ).
Geographic Shifts: Outbreaks continued to occur in traditional high-risk regions (West and Central Africa) but also emerged in new areas, including East Africa (South Sudan, Kenya, Ethiopia).

Significance and Claims
The authors assert that high morbidity and mortality from YF persist in Africa despite the EYE strategy's implementation. The study concludes that while there have been modest improvements in the timeliness of RVCs and vaccination rates, these changes are not statistically significant, and substantial delays (often >100 days) remain a critical barrier to outbreak control.

The paper claims that the observed persistence of large outbreaks and high CFRs likely reflects a combination of factors, including gaps in diagnostic networks, vaccine supply constraints, and the changing epidemiology of YF into new regions, rather than a failure of the EYE strategy itself. The authors caution that the reduction in CFR may be partly attributed to increased diagnostic capacity detecting milder cases.

Ultimately, the study emphasizes that achieving YF control requires continued prioritization of robust response mechanisms. The authors state that future success depends on increasing baseline vaccination coverage, further decreasing the time to reactive vaccination, and developing therapeutic options, particularly as the EYE strategy transitions in 2026. The results are presented as descriptive, acknowledging limitations in surveillance variability and case reporting biases, rather than as a definitive causal assessment of the EYE strategy's impact.

Technical Summary: Yellow Fever Outbreaks and Reactive Vaccination in Africa, 2010–2025

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