As an epidemiologist who works with mathematical models to analyze outbreaks, I can tell you that, despite the news from Dallas of the first U.S. case of Ebola, there is some good news from the Ebola outbreak ravaging West Africa. This Ebola is not spreading nearly as fast as some past scourges.
The distinction of being the fastest-spreading disease in human history goes to measles. In the era prior to 1963, when children were first routinely vaccinated, each case of measles created 17 secondary cases, with transmission spreading like wildfire in schools, especially. It was lethal in 1 out of every 3 to 4 cases. At this rate, getting infected with measles during childhood was inevitable, and so was death for too many.
Of course, reproductive rate isn’t the whole story. Each case of the Spanish flu, which caused the pandemic of 1918-1920 that many call the worst in recent history, produced two to five additional victims. While that’s much lower than measles, Spanish flu was still able to spread worldwide because of the speed at which it moved.
Only two to three days elapsed between the first case and a generation of secondary cases. A substantial number of transmissions can occur even before a person realizes that he or she has the flu. Another factor is that flu — like measles, but unlike Ebola — can be spread through the air, making it only a cough or a sneeze away from infecting a new host. An estimated 30 million to 50 million people around the world died from the Spanish flu.
The good news is that Ebola has a lower reproductive rate and a longer generation interval than the Spanish flu and a much lower reproductive rate than measles. Work that collaborators and I published in 2004, which produced the first estimates for Ebola’s reproductive rate by using data from outbreaks in Central Africa in 1995 and 2000, found that each case of Ebola produced 1.3 to 1.8 secondary cases on average.
A colleague and I recently found (our findings are at http://bit.ly/1lY2pto) that the reproductive rate of the current outbreak is about the same. It hasn’t become more transmissible in the more than 10 years it was lying low — and humankind has experience in dealing with it.
The time that elapses between the first Ebola case and the generation of secondary cases — about two weeks — should allow plenty of time to identify those who are sick and protect people who might come in contact with them.
To break the chain of the current outbreak, our numbers show that health care workers need to halt about 50 percent of infectious contacts by effectively isolating people who are infectious. Working against us is the fact that the countries suffering from outbreaks lack gloves, gowns, face masks and other essential supplies to protect nurses and doctors from infection. They also don’t have an adequate surveillance system to catch Ebola cases in a timely way. The number of doctors and health centers is small as well. As a result, Ebola has spread with very little resistance.
Up until this year, textbooks have described Ebola as the type of infectious disease that could cause no more than a few hundred cases in hard-to-reach forested areas of Central Africa. But while the virus has not changed much in more than a decade, many parts of Africa are very different. People live in much denser quarters, and the population is also much more mobile than before. This increases opportunities for the virus to reach new areas where new chains of transmission can grow and become established.
Math and history show us that decisive efforts to isolate those who are infected with Ebola and to follow up quickly with the potential contacts of the infected can help to get an outbreak under control. We’re lucky that we have such capacities in the U.S.; even with Ebola now present here, it should not gain much of a foothold. But our world is interconnected in ways it never was before, and diseases that require substantial contact to spread aren’t the only things circulating. If a virus like Ebola can quickly flare out of control, consider the impacts of a novel strain of influenza.
Gerardo Chowell-Puente is associate professor of mathematical epidemiology at the School of Human Evolution and Social Change at Arizona State University. He wrote this for Zocalo Public Square. Reach him at gchowell@asu.edu.
DISEASE TRANSMISSION
The speed of an epidemic’s spread is gauged by its basic reproduction number. At its essence, the figure represents the expected number of new cases spread by each person who gets sick. The larger the number, the more difficult it becomes to control the spread of a disease. On that basis, and because Ebola cannot be transmitted via airborne droplet, doctors are optimistic about their ability to control the virus.
Here are some notable infectious diseases, with their modes of transmission and estimated reproductive rates:
Measles (airborne)12-18 cases
Polio (fecal-oral)5-7
Smallpox (airborne droplet)5-7
Influenza of 1918 (airborne droplet)2-5
Ebola (bodily fluids)1-2
SOURCES: Centers for Disease Control and Prevention; Gerardo Chowell-Puente
