The Two Numbers That Tell You Whether the Ebola Outbreak Is Under Control

When public health officials say an Ebola outbreak is coming under control, they are making a claim that can be checked, and two measurements do most of the checking. Anyone following the Bundibugyo virus outbreak now spreading through Ituri, North Kivu, and South Kivu in the Democratic Republic of the Congo, with imported cases already confirmed in Kampala, can use these two numbers to see past case counts and judge whether the response is working. I went looking for both numbers in the situation reports published by Africa CDC, the World Health Organization, the U.S. Centers for Disease Control and Prevention, and Uganda’s Ministry of Health, and I could not calculate either of them with any confidence. That’s a problem.

Knowing Where the Next Case Will Come From

The first number is the share of new cases that were already on a list generated by contact tracing. In an outbreak that is genuinely under control, nearly every person who falls ill was already known to the response, identified earlier as the contact of a confirmed case and placed under daily monitoring precisely because the system anticipated they might become sick. The goal is a closed loop, in which the only new cases of Ebola are people whose exposure was traced, whose names were recorded, and whose first symptoms were caught quickly because someone was already watching for them. The moment a person develops Ebola with no place in any known chain of transmission, that single fact tells you the virus is moving through the population faster than the response can follow it, and that there are exposures happening out of sight.

This is why the proportion of cases arising from listed contacts is the closest thing field epidemiology has to a scoreboard. It measures the one thing that actually breaks transmission, which is the ability to find and isolate people before they expose others. A response can deploy treatment beds, ship in personal protective equipment, and hold press briefings, and still be losing if most new patients keep appearing from nowhere. Building and sustaining that capacity is structural work; it depends on a trained corps of contact tracers, functioning laboratories that can confirm cases within hours, community trust deep enough that people will name their contacts honestly, and the logistics to monitor those contacts daily across remote and insecure terrain. Where any of those foundations is weak, the percentage of cases from known contacts stays low, and no amount of individual diligence by frontline workers can fully compensate.

Reading the Case-Fatality Rate the Right Way

The second number is the trajectory of the case-fatality rate, and reading it correctly requires resisting the most natural mistake. Early in any outbreak, the recorded case-fatality rate runs extremely high, because the only patients the system detects are the ones sick enough to seek care or die, while the milder and earlier cases go uncounted. As case finding improves and surveillance reaches further into the community, that rate should fall, not because the disease has grown less lethal but because the denominator finally includes the people who were always there. A case-fatality rate that drifts downward over successive weeks is therefore one of the clearest signals that detection is catching up to the epidemic.

For that signal to be legible, officials have to report the case-fatality rate by cohort, tracking the outcomes of everyone diagnosed in a given week rather than collapsing every case since the outbreak began into a single cumulative figure. A cumulative number blends the undetected early period with the better-surveilled present and obscures exactly the trend that matters. This is not an academic refinement. With Bundibugyo virus there is no licensed vaccine and no specific treatment, and survival turns heavily on how early a patient reaches supportive care. A falling cohort case-fatality rate would be direct evidence that people are being found sooner; a flat or rising one, week after week, would say that they are still arriving too late, or not arriving at all.

The Absence of These Numbers Is a Finding in Itself

The official reports are not short on numbers and graphs. They tally cumulative confirmed cases, deaths, affected health zones, and isolation beds, and those totals climb with each update. What the cumulative totals cannot tell you is whether the response is gaining ground, because a number that only ever rises looks the same whether transmission is accelerating or being steadily extinguished among people already under watch. The two indicators that would answer the question, the proportion of cases from listed contacts and the cohort trend in case-fatality, are the ones I found hardest to extract. We need these reports to also include the most decision-relevant measurements.

That this outbreak has erupted at a moment when global health surveillance system budgets have been cut sharply only raises the stakes, because the capacity to generate the right indicators is the same capacity that funding reductions erode first. An outbreak is brought under control when officials can say, for every new case, who infected that person and whom that person exposed. Until the situation reports let us watch that number climb toward one hundred percent, the most important question about this epidemic remains unanswered, and we should be worry that we are flying with the instruments we most need switched off.