Thirty-one people are now dead and dozens more are sick as the latest Ebola epidemic is trickling through central Uganda. Cases are rising slowly, but the epidemic has spread to seven districts including the capital, Kampala. What happens next is difficult to predict.
What is clear is the need for a global effort to prevent the worst. That effort has now begun. The Uganda Ministry of Health is at the center of the effort with support from the Centers for Disease Control and Prevention, World Health Organization, and other government agencies. The question is whether these interventions are in time to avert a large outbreak?
The good: slowly rising cases
Since the first death in September, there have been 30 deaths and 109 confirmed cases of Ebola virus disease in Uganda. The Uganda Ministry of Health reported the 14 newest confirmed cases on October 26, along with twenty probable cases.
The World Health Organization Situation Report #35 shows the increase over time:
Cases have continued to increase since the last official report, however. On October 27, the Ministry of Health tweeted that twelve new cases had been found in the Kassanda district of Central Uganda, 100km west of the capital city Kampala.
The good news is that cases are not rising fast. This is partly due to the fact that Ebola does not typically generate explosive outbreaks (in contrast to, for instance, SARS-CoV-2, the virus that causes Covid-19).
One reason that Ebola outbreaks tend to simmer is that the virus has a relatively long incubation period, the time between when a person is infected and when they begin to show symptoms. For Ebola, it typically takes 5-12 days to show symptoms. This then means that the serial interval, the time between the onset of illness in one generation of the infection and the next is also long — about two weeks. So, compared to respiratory diseases like Covid-19 and flu, Ebola epidemics are relatively slow-burning.
The bad: spatial spread from the epidemic epicenter
The bad news is that the disease has reached seven districts, including the capital city of Kampala. Kampala has a high population density and is home to almost 1.7 million people. The first case in Kampala was reported by the BBC on October 11, 2022, although it is believed that this case did not originate in the city. Since then, at least sixteen additional cases have been reported, mostly in the city’s western Rubaga Division.
It is difficult to say what will happen next. Transmission appears to be leveling off. It is tempting to think that control efforts have resulted in a slowing of transmission: the Ministry of Health and numerous international partners have mobilized to provide emergency services and clinical care, to perform safe and dignified burials of people who have died from the virus, to engage communities, and to perform contact tracing. A cordon sanitaire has been established in several affected areas.
However, Ebola outbreaks tend to be quite “bursty”, especially at the beginning, so the slowing we are seeing in recent days may not be real. This burstiness results from two things. The first is the long serial interval, which means that days or even weeks of relative quiet may occur, to be interrupted later by new clusters of infection. The second is that Ebola is prone to superspreading. Superspreading occurs because the number of secondary cases that arise from any primary infection is highly variable. While most Ebola patients do not infect more than one other person and many do not infect any others at all, a small fraction of cases give rise to a large number of secondary cases, continuing the chains of transmission that keep the epidemic going. A long serial interval and high level of superspreading give rise to bursty epidemics.
Bursty epidemics may be hard to predict, but they are not harder to contain than other epidemics. The keys to containing bursty epidemics are vigilance, coordination, and rapid response, which the Uganda Ministry of Health clearly knows. You can follow them on twitter at @MinofHealthUG.