major ebola outbreaks courtesy of bloomberg-com

The ominous math of the Ebola epidemic – The Washington Post

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 1.5 to two.

Ebola cases in West Africa have been doubling about every three weeks, and officials have presented no evidence of a major change in that trendline.

“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.

The latest World Health Organization statistics, published Wednesday, show 8,033 cases of suspected or confirmed Ebola in the West Africa outbreak, with 3,865 deaths. That figure does not include the death Wednesday of Thomas Eric Duncan, a Liberian man who died in Dallas.

“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.

There is little sign that the intervention is having an effect. The U.S. military is building 17 treatment centers that can hold 100 people each, but the top military commander in Africa said Tuesday that they won’t be ready until mid-November.

“The virus is moving on virus time; we’re moving on bureaucracy or program time,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “The Ebola virus is actually picking up the pace. Even as we add resources, we get farther behind.”

The latest surveillance data from the WHO shows only faint hints of progress in bringing Ebola under control in West Africa. The number of infections is dropping in several areas, according to the organization, but in many places the numbers are exploding. Experts caution against reading too much into small fluctuations that may simply reflect an increase or decrease in surveillance or a reappraisal of older data.

“It’s kind of like if I asked you what the weather is going to be like in D.C. on February 14. The best thing I can probably say is, ‘It’s going to be a little cold.’ That’s where we are right now,” Osterholm said. “This is an evolving situation where the facts and information of yesterday might not be the facts and information of tomorrow.”

Gerardo Chowell, a mathematical epidemiologist at Arizona State University, used data compiled through the end of August to estimate the reproduction number of 1.5 to two for this Ebola epidemic. Chowell said that even modest gains in lowering that number could give health officials and the military a better chance of controlling the epidemic.

“Maybe we can bring it from two to 1.2 or 1.3, which would indicate that the number of new cases will be dramatically reduced, and that will give you time,” he said.

Another key number: how many days elapse between the time symptoms occur (which is when a person becomes contagious) and when health officials diagnose the disease in that person. Driving that number down is critical to containing the virus.

The incubation period for Ebola is usually about a week to 10 days, although it can last as long as 21 days. That creates obvious challenges for health workers who have to do contact tracing — they have to repeatedly knock on doors and take the temperatures of people who weeks earlier were potentially exposed to the virus. But it also gives those same workers a decent interval of time to track down people who may be infected before they start shedding the virus and potentially spreading the disease.

There are several scenarios for how this plays out. One is that the conventional methods of containing Ebola — isolating patients and doing contact tracing of people who might be exposed — lower the rate of new infections until finally the epidemic burns itself out. That has been the case in all previous outbreaks of Ebola and similar viruses, although no outbreak has ever been nearly as extensive as this one, in which the virus jumped into urban areas for the first time.

A second scenario is more dire: The conventional methods come too late, the epidemic keeps spreading, and the virus is beaten back only when new drugs or vaccines can be developed and scaled up to the point where they can be widely distributed.

There are other, very low-probability possibilities, such as Ebola mutating to become airborne. Despite much chatter about this, there is no evidence of it, according to scientists who study Ebola and who, in some cases, have scrutinized this strain. In human history, no virus that affects people has changed its method of transmission. It’s not impossible, as Frieden noted in a news conference, but the cases in this outbreak have followed the usual pattern — caregivers, close family members and health-care workers have accounted for the overwhelming majority of cases.

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?”

Anthony Fauci, director of National Institute of Allergy and Infectious Diseases, said in an interview that, even if the international community accelerates its response, “it’s conceivable that we will not be able to control it if we don’t put enough resources in. We might need to rely on a safe, effective vaccine.”

Researchers at NIH institute began a trial involving 20 people in early September to determine the safety of an Ebola vaccine that was already in the development pipeline. The initial results should be available later this fall.

The next step would be a larger human trial beginning next year, perhaps involving 10,000 to 20,000 people in West Africa, to determine whether the vaccine works. Only then would it be safe and ethical to distribute it widely.

By Joel Achenbach, Lena H. Sun and Brady Dennis

Reblogged via The ominous math of the Ebola epidemic – The Washington Post.


What did you think about this? Please leave a reply.

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.