The findings could help scientists develop a vaccine to protect drug users and other groups most at risk of contracting the disease.

About four million people in the United States and 170m people worldwide have been infected with the hepatitis C virus (HCV), for which, unlike hepatitis A and B, there is no vaccine.

About 85% of people infected with HCV develop persistent infection and are at risk of long-term complications, including liver cirrhosis and liver cancer.

David Thomas and colleagues from Johns Hopkins School of Medicine in Baltimore, USA, aimed to establish whether similar immunity could be achieved in people, according to an article in The Lancet medical journal.

They selected injecting drug users and identified 164 who had no evidence of previous HCV infection and another 98 who had been previously infected with HCV, but had cleared the infection.

The incidence of HCV infection was halved for people who had been previously infected, compared with those who had not been previously infected (12% and 21% respectively).

Creative research

David Thomas said: "The high rate of HCV infection in injecting-drug users underscores the importance of preventing HCV infection.

"Since it appears that immunity can be acquired to protect against viral persistence, vaccines should be tested to reduce the medical consequences of HCV infection among people at highest risk."

Hepatitis C causes jaundice, fatigue, abdominal pain, nausea and loss of appetite.

David Grant, from the Memorial University of Newfoundland, Canada, said: "While the most positive interpretation of this unique study offers hope that protection against HCV can be acquired, the immunogenicity of human vaccines still pales compared with that of genuine infections.

"The need for continued creative research in vaccine design is emphatically underlined by the, at best, part-protection against persistent secondary infection conferred by clearance of primary infection with HCV itself."

The British Liver Trust is optimistic about the research.

The trust's chief executive Nigel Hughes said: "We don't fully understand the mechanisms of immunity.

"This is an interesting development and I would hope more studies are done and it's replicated on a wider scale.

"This could kick-start the research even more.

"We desperately need a vaccine."