For the first time, there is evidence that a vaccine protects people from Ebola. The interim study results, unveiled by the World Health Organization on July 31 and published online by The Lancet, suggest that a single shot can protect people who were directly or indirectly exposed to individuals with the disease. The vaccine, which appears safe, could fundamentally alter how well humans contain future Ebola outbreaks. Already, the current epidemic has led to more than 11,000 deaths and 27,000 cases.

This candidate vaccine was being tested in people living in Guinea and is based on an animal virus that primarily affects rodents, cattle, swine and horses. One gene of that virus, however, was replaced with a segment of the gene for the outer protein of an Ebola virus species. Researchers at the Public Health Agency of Canada developed the immunization, which is now being produced by pharmaceutical company Merck, based in New Jersey.

The only individuals in the trial were those who had been exposed to newly identified Ebola patients or were those contacts’ contacts. This unique study approach, called “ring” vaccination, is based on the smallpox eradication strategy. It effectively forms a “ring” of protection by targeting individuals directly around the infected person and the people around those contacts’ contacts as well. For the trial, 90 groups, which included thousands of people, were randomly sorted to either have the whole ring around a contact be immediately vaccinated or, alternatively, a person’s contacts and contacts’ contacts would all be vaccinated after 21 days (the tail end of the accepted Ebola incubation period). Each of the 90 groupings would thus include all the contacts and the contacts’ contacts of an Ebola patient. The idea was to compare the number of cases in the immediately vaccinated versus with those who were delayed, Then researchers could assess if the vaccine was working. The interim data show that not a single one of the 2,014 people who were immediately inoculated developed Ebola after 10 days of their vaccination.

In contrast, of the 2,380 eligible individuals in the delayed vaccination group, which functioned as the control group, 16 developed Ebola. The preliminary finding suggests that the vaccine protected everyone in the immediate vaccine group against disease, shaking out to 100 percent efficacy. Based on the promising results, WHO is no longer delaying anyone from receiving the vaccination for the purpose of the study. Instead, Ebola contacts and contacts’ contacts will be vaccinated and researchers will continue to gather data. It is still unknown how long the vaccine may protect individuals for and if a booster shot would be needed.

Scientific American spoke with Anthony Fauci, director of the National Institute for Allergy and Infectious Disease, to find out more about what this promising new finding will mean for thwarting Ebola in the future. Fauci was not involved with the trial.

[An edited transcript of the interview follows.]

WHO announced that the vaccine appears to be 100 percent effective but that it might end up being around 75 percent effective, in reality. What does that mean?
The sample size was too small to definitively say it’s 100 percent effective. That would be unprecedented. There has never been a vaccine that is 100 percent effective. One hundred percent effective would mean everyone vaccinated would never be infected—and that doesn’t happen. This is a highly effective vaccine though. 

You address Ebola with a combination of things. Good public health preventative measures like identifying cases and contact tracing will still be important for preventing and controlling outbreaks. When you have an effective vaccine that adds an important tool to address and combat Ebola.

What does having this vaccine mean for future outbreaks?
A vaccine is helpful in controlling outbreaks. If you had a vaccine that was effective in the beginning of the outbreak in west Africa, it likely would’ve saved a lot of deaths and prevented a lot of suffering. 

How does this vaccine work? Is it like other vaccines out there, say for polio?
The fundamental principles of vaccines all have a common denominator: they induce an immune response that will hopefully protect the person. There is no live Ebola virus in this vaccine. The live virus is a harmless animal virus called the vesicular stomatitis virus (VSV). The VSV is what is called the vector carrier virus. You insert a single gene of Ebola into the VSV and when the VSV is injected into a person it replicates and makes Ebola protein, which stimulates the recipient to mount an immune response and make antibodies against Ebola. 

With the cases of Ebola thankfully falling so low—there were seven new cases last week—how could further testing take place?
It won’t. It’s very unlikely that further testing would be able to completed. That’s why several other vaccine candidates are sitting in the queue waiting to be tested. It’s very difficult if not impossible to complete testing of them. It’s the same with this vaccine. 

The next step will be if there is another outbreak, to use it and try to obtain more data as it is used.

Would there be a control group then that would not receive the vaccine?
That would be difficult to do. You could do a noninferiority test to compare it against another vaccine, but it’s difficult when you have a vaccine that is shown to be effective to have a complete placebo trial. 

What does this mean for the U.S.? Is there any talk of establishing a small stockpile of the vaccine in case there is a case of Ebola in the U.S. in the future?
I doubt very seriously there would be a vaccine used in the U.S. The chances of there being an outbreak in the U.S. are extraordinarily low. You might want to vaccinate workers going to an area with outbreaks, but talking about vaccinating the American public in general? That kind of thing would not likely happen. 

Will the vaccine need to be licensed in the U.S. for any use in health care workers?
I think what would happen is the company that sponsored it will have to submit data to the [U.S. Food and Drug Administration] and then the FDA would decide if it is licensable. If it is, then there could be production of it if needed, particularly for health workers. 

How quickly was this vaccine developed?
Vaccines generally take years and years to develop. This, too, wasn’t developed overnight. There were a couple vaccine candidates. There was one being tried in Liberia that was in development for 10 years but trials could not be completed because there were so few Ebola cases. This one we’re talking about today, the VSV, was developed in a few years. 

Did the Ebola outbreak in west Africa speed along its progress?
You can’t fully determine the efficacy of a vaccine unless you test it in the field when there are infections going on. In that sense it was fortuitous that that stage of development could be tested right at the time there was an outbreak.