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The trials involved two candidate Ebola vaccines that were given to volunteers in separate shots, the researchers said. One vaccine was provided first as a "primer" shot, and the second was given as a "booster."
In both cases, the viruses had been altered to include genetic material from Ebola, so any immune response to the vaccine could theoretically promote immunity to Ebola, the study authors explained.
An immune response was observed after primary immunization with the AD-26 vaccine, and boosting with the MVA resulted in sustained elevation of specific immunity to Ebola, the researchers found.
Further, neither vaccine caused any serious adverse events in the 87 people, aged 18 to 50, who volunteered for the phase 1 clinical trial.
The findings were published April 19 in the Journal of the American Medical Association.
The recent West African outbreak of Ebola, the largest in history, killed more than 11,000 people and infected more than 28,000, according to the U.S. Centers for Disease Control and Prevention.
Ebola expert Dr. Lee Norman, chief medical officer for the University of Kansas Hospital, said it's good that researchers are investigating different ways of delivering vaccines for the deadly virus.
"The delivery method, if it is successful, is really an important building block for a successful vaccine," Norman said. "We have to remind ourselves that phase 1 trials are typically more about safety than about effectiveness. It isn't typically the goal of a phase 1 trial to emphasize effectiveness."
Dr. Amesh Adalja, a senior associate at the UPMC Center for Health Security in Baltimore, agreed.
"Since the West African Ebola outbreak, many different vaccine approaches have flourished, and with several promising candidate vaccines the manner in which future Ebola outbreaks will be managed will be dramatically improved," Adalja said.
"Future studies will be needed to compare approaches and determine which are the most effective, but each new vaccine approach is a welcome tool the world did not have when the West African outbreak began," he added.
However, another Ebola expert questioned how useful a two-step vaccine would prove during a rapidly spreading outbreak.
"In a real-world scenario, you don't really have time for a prime/boost," explained Thomas Geisbert, a professor of microbiology and immunology at the University of Texas Medical Branch. "You really need a single injection that's fast-acting and works quickly."
Geisbert also said that in this clinical trial, people were provided the needed "booster" shot either 28 or 56 days after receiving the "primer" vaccine.
"You don't have that kind of time in an outbreak," he said. "You need something that's works in a week or two."
Worse, 40 percent to 60 percent of people in sub-Saharan Africa have low to moderate immunity against AD-26, according to the researchers. "That could definitely interfere with the effectiveness of the vaccine," Geisbert said.
Other vaccines are further along in the clinical trial process and have shown much more promise, particularly rival American and Canadian vaccines that were tested in Africa at the height of the Ebola outbreak, Geisbert said.
Despite these drawbacks, Norman said the results from this early trial will give Ebola vaccine hunters some valuable data.
"We have to remind ourselves that scientific discovery is not usually one study and an 'aha' moment where all questions are answered. Everything builds on everything else," Norman said. "We don't have anything now that's commercially available. The way I see it, let's keep working at it."
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