A cousin of the common cold virus offers the first hope of an effective vaccine against the chronic liver infection hepatitis C, it has been revealed.
Early trial results mirror responses seen in the minority of people with natural immunity to the disease, which can lead to severe liver damage and early death.
Scientists are encouraged by the findings but caution that it may take several more years to develop a fully approved vaccine.
Professor Paul Klenerman, from the Nuffield Department of Clinical Medicine at Oxford University, a leading member of the research team, said: "We've found that it's possible to prime large cellular immune responses against hepatitis C that last for at least a year.
"The immune responses we've seen are exciting and we are beginning the next stage of trials. While we are hopeful, it could be a long road to any vaccine that protects people against hepatitis C."
Hepatitis C is caused by a virus that can remain hidden in the bloodstream for decades before showing any symptoms.
Its effect is insidious, producing mild flu-like effects while inflicting slow damage to the liver which can lead to cirrhosis and cancer.
An estimated 200,000 to 500,000 people are infected with hepatitis C in England and Wales. Because of the lack of symptoms, many do not know of their condition.
The virus is carried in the blood and to a lesser extent other bodily fluids. It is most commonly transmitted by sharing needles to inject drugs, but can also be passed on via toothbrushes, razors, scissors, tattoos and body piercing.
There is a low risk of the virus passing from mother to child or between sexual partners.
Like HIV, hepatitis C presents a difficult moving target to anyone attempting to challenge it with a vaccine.
The virus can easily adopt new disguises, donning different molecular "coats" that may not be recognised by the immune system. It also comes in six possible strains, each presenting a different vaccine target.
For these reasons to date there has been no available vaccine for hepatitis C.
The new trial results, published in the journal Science Translational Medicine, suggest one may now be within reach that offers long-term protection.
A group of 41 healthy adults took part in the Phase One study primarily designed to test safety and obtain dosing information.
The vaccine is designed to provoke a response from immune cells called T-cells that target the inside of the virus rather than its outer coat.
It was created from a modified adenovirus, a member of a family of viruses responsible for the common cold and sore throat infections.
Two kinds of adenovirus were tested as vaccine candidates, a rare human version and one found in chimpanzees.
Adenoviruses are known to stimulate T-cells and can be altered to prevent them replicating and becoming infectious in their own right.
"The outside shell of the hepatitis C virus is very variable but the inside of the virus is much more stable," said Prof Klenerman.
"That's where the engine of the virus is, where we may be able to successfully target many of the crucial pieces of machinery. But we need T-cells and not antibodies to be able to react to the inner components of the virus."
The vaccine triggered a large T-cell response that lasted for at least a year, the length of the trial. No significant adverse effects were reported among the volunteers.
The immune response was of a similar type and size as that reported in people who naturally clear the hepatitis C virus from their bodies, said the scientists. Around one in five people are naturally immune to hepatitis C infection.
Not only did the T-cells react to a wide range of different elements within the virus, but they seemed to respond to more than one strain.
The Oxford team has now launched another trial to see if the vaccine can help treat those already injected with hepatitis C.
"T-cell responses often become weak in those with chronic hepatitis C infections," said Prof Klenerman. "It may be that using a vaccine to boost their immunity could become part of any treatment with other drugs."
A US team is looking to carry out a larger trial of the vaccine involving at-risk groups.
Charles Gore, chief executive of the Hepatitis C Trust, said: "This is very promising research. There has been rapid development in drugs to treat hepatitis C but vaccine development has lagged behind. Yet, if we only treat existing infections, we will always be behind the curve. We badly need to improve prevention and this is an excellent step in that direction."