Scientists at Oxford University have begun immunising healthy volunteers with a new Ebola vaccine.
In September last year a separate trial of another Ebola vaccine got under way in the city.
This latest trial involves 72 volunteers aged 18-50.
Initial tests in monkeys showed the vaccine, developed by Janssen Pharmaceutical Companies of Johnson and Johnson, gave complete protection against Ebola.
The volunteers in Oxford are the first humans to receive the vaccine.
Dr Matthew Snape, from the Oxford Vaccine Group, part of the University of Oxford Department of Paediatrics, said: "We aim to immunise all participants within a month.
"The main aim is to understand the safety profile of the vaccines."
The trial involves volunteers receiving an additional booster dose one or two months after the initial injection.
Similar small trials will also get under way in the US and three African countries unaffected by Ebola.
The first dose is designed to prime the immune system with the second booster dose to enhance the immune response.
The two doses contain different components, but both include genes for a protein from the Zaire strain of the Ebola virus.
The trial organisers stress the vaccine cannot cause anyone to be infected with Ebola.
The immune response the vaccine generates - both antibodies and T cells - will be measured over the course of a year.
Johnson and Johnson said it hoped to begin a larger Phase 2 trial in Africa and Europe within three months and then to have the vaccine available for use in Liberia, Guinea and Sierra Leone by the middle of 2015.
The company says it could have 2m doses of the vaccine available this year.
Other Ebola vaccines
The health crisis triggered by the Ebola outbreak has led to a huge acceleration in the pace of vaccine research.
In September a separate team at the Jenner Institute in Oxford began a trial of an Ebola vaccine.
GlaxoSmithKline and the National Institutes of Health in the US developed the vaccine.
Results of that trial are due shortly and there are plans for the vaccine to be offered to health workers in Ebola affected countries of west Africa later this month.
The study led by the Jenner Institute has now been modified to include a booster dose.
Merck has recently bought the rights to a third Ebola vaccine being developed by the biotech company NewLink Genetics.
That vaccine is being tested in Switzerland.
The trial, in Geneva, was halted last month after some volunteers complained of joint pain.
It resumed this week, with participants being given a lower dose.
Ebola vaccines are also being developed in Russia.
Dr Snape said: 'The fact that there are at least three Ebola vaccines entering these early safety trials is good news.
"We are not playing first past the post here. Having multiple vaccines progressing through clinical trials increases the likelihood of vaccine manufacturers having the capacity to meet production demands should mass immunisation be required.
"The more vaccines and more manufacturers there are working on this, the better.'