NEW YORK (Reuters Health) - Some people who become infected with the hepatitis C virus but clear the infection may develop an immunity that protects them the next time they are exposed to the virus, the results of a new study suggest.

Even though researchers found an "alarming" rate of hepatitis C infection in injection-drug users in the study, the findings raise the hope that it may be possible to develop an effective vaccine for hepatitis C.

Nearly 4 million Americans have hepatitis C, making it the most common chronic viral infection in the US. Chronic inflammation of the liver develops in many patients, and about 20% of people with hepatitis C will develop cirrhosis, a severe and sometimes fatal scarring of the liver. Cirrhosis increases the risk of liver cancer.

Hepatitis C is spread through contact with blood and other body fluids, but the route of transmission remains undetermined in a substantial percentage of infections. People who share needles to inject drugs have a high risk of contracting the disease.

In a study of injection-drug users in Baltimore, Maryland, a team led by Dr. David L. Thomas and Shruti Mehta from the Johns Hopkins School of Public Health used blood tests to identify 164 people who had never been infected with the virus and 98 who had been infected but had cleared the infection.

The researchers followed these participants for a little more than 2 years to see whether they contracted hepatitis C again. During the study, all participants were counseled to reduce their drug use and were referred to treatment, although many continued to take drugs.

Of participants who had not been infected previously, 21% became infected with the hepatitis C virus during the study period. In contrast, 12% of participants who had already cleared a hepatitis C infection in the past became infected again. The results of the study are published in the April 27th issue of the medical journal The Lancet.

"Those who previously recovered from infection and were then infected again often resolved the new infection, suggesting that immunity could be developed that promotes recovery," Thomas told Reuters Health.

"This is important because prior studies that showed that you could be reinfected cast doubt on the prospects of developing an effective vaccine," Thomas said. Although reinfection did occur in the study, these infections usually cleared and would not be expected to cause disease, according to the Johns Hopkins researcher.

Whether or not a person was infected with HIV, the virus that causes AIDS, had a major impact on the odds of getting infected with hepatitis C, the report indicates. People who were not infected with HIV were 12 times less likely to develop a lasting case of hepatitis C than people who were HIV positive.

Based on the results of the study, Thomas and his colleagues recommend that vaccines should be tested in people at the highest risk of getting infected with hepatitis C. Vaccination has been only mildly effective in chimpanzees, but ineffective in humans.

However, it is difficult to be certain whether the patients who had already been exposed to hepatitis C virus and later cleared another infection actually developed an immunity to the virus, Dr. Michael Grant at the University of Newfoundland in Canada told Reuters Health. He pointed out that not all of the patients who had been previously exposed had such an immunity the second time around.

"Although the research indicates that protective immunity is possible, it also illustrates that it can be tenuous and that a good protective vaccine will be difficult to make," Grant said.

SOURCE: The Lancet 2002;359:1452, 1478-1483.

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