Immunity and Vaccination
Second cases of type D hepatitis have not been reported in humans. This indicates that acute, self-limiting HDV leads to immunity. A study in chimps found that the chimps that were deliberately superinfected with HDV resisted rechallenge with HDV 6 months later but were venerable 3 to 5 years after initial HDV infection. There is evidence in woodchucks that immunity to epitopes of the internal HDAg antigen of HDV modifies HDV infection. This is analogous to how vaccination against the internal HBcAg antigen of HBV has been shown to protect against HBV. This mechanism for this immunity appears to be cell-mediated. Humoral immunity to the antigens of HDV or HBV is not enough to protect against infection. This is known because both viruses can be perinatally transmitted to infants, who have a high level of maternal antibodies.
Since HDV requires HBV coinfection in order to replicate, the HBV vaccine is effective for HDV. Luckily, the vaccine for HBV is very effective and is currently available. Unfortunately some of the most high-risk populations are also the most difficult to reach.
Over 300 million people in the world who are chronically infected with HBV are also at risk of contracting HDV. Since they are refractory to Hepatitis B vaccine, they would need a separate vaccine to protect against HDV superinfection. Experimental studies in animals are currently underway to see if HDAg can give protection against HDV superinfection or slow the liver disease in these carriers.