Ebola is too unpredictable. The last outbreak occurred in 2012 in the Democratic Republic of Congo, which is 2,500 miles from Sierra Leone, the heart of the current epidemic. Short of vaccinating the entire continent of Africa, there’s no good way to predict who is going to get the disease next.

“We don’t know where the next outbreak is going to be,” Kartik Chandran, an associate professor of microbiology and immunology at Albert Einstein College of Medicine, told me. “It strikes like lightning and then disappears.”

There are several strains of Ebola. The current strain is ZEBOV, or Zaire virus, but there are also Sudan and Cote d’Ivoire versions. A vaccine would have to work against all of them.

Vaccines don’t work that well in fast-moving epidemics. There are a few things you can do with a vaccine once an outbreak starts. One is immunizing healthcare workers and the families of infected patients. Sometimes doctors try “ring vaccination,” or targeting residents of villages on the perimeter of the outbreak in an attempt to isolate and quash it.