The Ominous Math of the Ebola Epidemic Share

When the experts describe the Ebola disaster, they do so with numbers. The statistics include not just the obvious ones, such as caseloads, deaths and the rate of infection, but also the ones that describe the speed of the global response.

Right now, the math still favors the virus.

Global health officials are looking closely at the “reproduction number,” which estimates how many people, on average, will catch the virus from each person stricken with Ebola. The epidemic will begin to decline when that number falls below one. A recent analysis estimated the number at 1.5 to 2.

The number of Ebola cases in West Africa has been doubling about every three weeks. There is little evidence so far that the epidemic is losing momentum.

“The speed at which things are moving on the ground, it’s hard for people to get their minds around. People don’t understand the concept of exponential growth,” said Tom Frieden, director of the U.S. Centers for Disease Control and Prevention. “Exponential growth in the context of three weeks means: ‘If I know that X needs to be done, and I work my butt off and get it done in three weeks, it’s now half as good as it needs to be.’ ”

Frieden warned Thursday that without immediate, concerted, bold action, the Ebola virus could become a global calamity on the scale of HIV. He spoke at a gathering of global health officials and government leaders at the World Bank headquarters in Washington. The president of Guinea was at the table, and the presidents of Liberia and Sierra Leone joined by video link. Amid much bureaucratic talk and table-thumping was an emerging theme: The virus is still outpacing the efforts to contain it.

“The situation is worse than it was 12 days ago. It’s entrenched in the capitals. Seventy percent of the people [who become infected] are definitely dying from this disease, and it is accelerating in almost all settings,” Bruce Aylward, assistant director general of the World Health Organization, told the group.

Aylward had come from West Africa only hours earlier. He offered three numbers: 70, 70 and 60. To bring the epidemic under control, officials should ensure that at least 70 percent of Ebola-victim burials are conducted safely, and that at least 70 percent of infected people are in treatment, within 60 days, he said.

More numbers came from Ernest Bai Koroma, president of Sierra Leone: The country desperately needs 750 doctors, 3,000 nurses, 1,500 hygienists, counselors and nutritionists.

The numbers in this crisis are notoriously squishy, however. Epidemiological data is sketchy at best. No one really knows exactly how big the epidemic is, in part because there are areas in Liberia, Sierra Leone and Guinea where disease detectives cannot venture because of safety concerns.

The current assumption is that for every four known Ebola cases, about six more go unreported.

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“This has been a particularly difficult outbreak because of the difficulty getting a lot of data quickly out of the countries,” said Martin Meltzer, a CDC researcher who models epidemics. “My crystal ball is painted a deep black. It’s like tracking a hurricane.”

Meltzer helped produce a report in late September that said that at current rates of infection, as many as 1.4 million people would become infected by January. That number, officials stressed, was a straight extrapolation of the explosive spread of Ebola at a time when the world had managed to mount only a feeble response. The more vigorous response underway is designed to bend that curve.

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As the number of infections increases, so does the possibility that a person with Ebola will carry it to another country. This is known as an export.

“So we had two exports in the first 2,000 patients,” Frieden said in a recent interview. “Now we’re going to have 20,000 cases, how many exports are we going to have?”