“All models are wrong, but some models are useful.” George E. P. Box (18 October 1919 – 28 March 2013) - Professor Emeritus of Statistics at the University of Wisconsin
#19,190
Late yesterday afternoon the CDC held an update (see transcript & video) on the the Ebola virus outbreak in Central Africa, focusing specifically on 3 new reports published Friday in the CDC's MMWR.
- The first is an Assessment of Risk to the U.S. Population from the Ebola Disease Outbreak Caused by Bundibugyo Virus, 2026 which finds `. . . the risk posed by this ongoing outbreak to the U.S. population during the next 3 months as low.'
- The second is Notes from the Field: Outbreak of Ebola Disease Caused by Bundibugyo Virus — Democratic Republic of the Congo and Uganda, May 2026 which reports `The scope of the outbreak is likely larger than that represented by available data and might prove challenging to contain and control.'
As with all such models, this isn't a prediction of what will be, only what might be, if certain things aren't done to prevent it.
Dr. PillaiCurrently, the situation is very fluid, and while the numbers are not completely known, based on the trajectory of the outbreak and the rapid extension into multiple different health zones over a short period of time, this appears to be in one of the lower end of the percentage of individuals that are being detected and isolated.
First, some excerpts from the report, after which I'll have a bit more.
Modeled Scenario Projections for the Ebola Disease Outbreak Caused by Bundibugyo Virus, 2026
Early Release / June 5, 2026 / 75
Eric Q. Mooring, ScD1,2; William T. Koval, PhD1; Isobel Routledge, PhD3; Inga Holmdahl, PhD4; Guido España, PhD1; Rebecca Kahn, PhD4; Beau B. Bruce, MD, PhD1
Summary
What is already known about this topic?
An outbreak of Bundibugyo virus disease (BVD), a type of Ebola disease, is currently ongoing, centered in the Ituri province of the Democratic Republic of the Congo (DRC).
What is added by this report?
CDC used a transmission model to project outbreak growth over 3 months, by using different assumptions about the number of deaths as of May 24, 2026, and by varying the percentages of persons with BVD who are successfully identified and isolated to prevent ongoing transmission. Assuming 50 cumulative deaths as of May 24, 2026, if 70% of patients were to enter isolation, only approximately one in 20 simulations projected an outbreak exceeding 10,000 cases within 3 months.
What are the implications for public health practice?
Large-scale, rapid public health action is needed to control the current outbreak, already the largest known BVD outbreak, from becoming one of the largest Ebola epidemics in history.
(SNIP)
Outbreak Size Projections and Inferred Spillover Date by Assumed Number of Deaths
Assuming 50 deaths. The model calibrated to 50 deaths estimated that the spillover event that triggered this outbreak most likely occurred on approximately February 19, 2026 (interquartile interval [IQI] = February 1–March 8). Assuming that 20% of infected persons were successfully isolated beginning May 24, 2026, projections showed ≥20,000 cumulative cases in 65% of simulations, ≥10,000 cumulative cases in 85% of simulations, and ≥4,000 cumulative deaths in 69% of simulations (Figure).
Even with 50% of infected persons isolated, many simulations still projected these numbers of cases but were less likely to occur (17% of simulations projected ≥20,000 cases and 22% projected ≥4,000 deaths). At 70% isolation, projected outbreaks were much more likely to be smaller, but still of substantial size, with 94% of simulations projecting <10,000 cases and only 1% projecting ≥20,000 cases; similarly, at this isolation level, 90% of simulations projected <2,000 deaths and only 3% projected ≥4,000 deaths. Re declined proportional to the percentage of infected persons successfully isolated (Supplementary Figure 1).
Assuming 100 deaths. Assuming 100 cumulative deaths as of May 24, 2026, the inferred median spillover date was February 8, 2026 (IQI = January 21–February 27). Very large outbreaks were likely in the scenario in which only 20% of patients were isolated (76% of simulations projected ≥20,000 cases and 87% projected ≥4,000 deaths). In the scenario in which 70% of infected persons were isolated, 73% of simulations projected <2,000 cumulative deaths by August 22, 2026, and 10% projected ≥4,000 deaths (Supplementary Figure 2).
Assuming 200 deaths. Assuming 200 deaths by May 24, 2026, the calibrated model inferred a median spillover date of January 29, 2026 (IQI = January 9–February 18). The earlier spillover date would have generated a larger outbreak by the time interventions began; thus, even with 70% of infected persons isolated, 42% of simulations projected ≥10,000 cases by August 22, 2026.
Sensitivity to Basic Reproductive Number
Simulated outbreaks with R0 values higher than the median R0 typically reached ≥10,000 cumulative cases and ≥2,000 cumulative deaths by August 22, 2026, in scenarios with ≤50% isolation, even assuming only 50 cumulative deaths by May 24. In the scenario with 70% of infected persons isolated and 50 assumed deaths by May 24, 2026, no simulations projected ≥2,000 deaths when R0 values were lower than the median R0, but 20% of simulations projected ≥2,000 deaths when R0 values exceeded the median (Supplementary Figure 3).
(SNIP)
The high probability of a large outbreak over a 3-month period primarily results from the large size of the outbreak at the time it was initially confirmed. This analysis did not provide evidence that R0 for this outbreak is unusually large.† Time between Ebola outbreak onset and detection is positively correlated with overall outbreak size and duration (4).
CDC’s assessment that the risk to the general U.S. population is low (5) is not changed by this analysis. Despite the unprecedented size of the 2014–2016 West Africa Ebola epidemic, only two Ebola transmission events occurred in the United States. Those two infected persons were health care workers caring for a patient with Ebola who had traveled to the United States before enhanced screening, risk assessment, and health education measures were implemented at U.S. ports of entry (6). Both persons infected in the United States recovered.
All three MMWR reports are worth reading in their entirety, and the video presentation (21 minutes) is very much worth watching.
While conditions could change, right now the DRC and surrounding countries appear to be on a trajectory that could eventually equal or even exceed that seen during the 2014-2016 West African Ebola Outbreak (28K cases, 11K deaths).
But, as they say, `If you've seen one epidemic . . . you've seen one epidemic.' The Bundibugyo virus is a different threat than Ebola Zaire, and the conditions in the DRC, Uganda, and South Sudan differ as well.
For now, this is 99% a regional threat. Exported cases are a possibility, but large outbreaks in places like Europe or North America - which are far better prepared to deal with this virus - are unlikely.
But all of this assumes that reasonable containment efforts are made - or even possible - at ground zero. Long chains of human-to-human transmission are problematic with any zoonotic virus, as it increases the chances that the virus will better adapt to a human host.
Which makes it very much worth our while to do whatever we can to help bring this outbreak under control.
Sooner, rather than later.
