The human immune response to HAV infection and vaccine studies in experimental animals both suggest that the virus is an ideal candidate for vaccine control. Natural infection with HAV appears to induce long lasting immunity. In addition, small doses of pooled human immune globulin are highly effective in preventing or ameliorating HAV infection in contacts of cases and in persons regularly exposed to known endemic settings (McCollum, 1982).
Studies in marmoset and chimpanzee models with both killed and live attenuated virus vaccines have been quite successful. Both vaccines induced neutralizing antibody against the virus; subsequently, the animals were totally protected against parenterally administered challenge virus (Provost et al., 1982, 1983).
While improved sanitation is an effective technique for reducing the incidence of HAV infection, and immune globulin administration can diminish or eliminate symptoms in exposed individuals, only a vaccination program will allow true control of the disease.
Defining the target population is the first step in calculating the benefit that could be produced by a vaccine candidate. This knowledge can be translated into an estimate for vaccine preventable illness (VPI). VPI is defined as the number of cases, complications, sequelae, and deaths that could be prevented by immunization of the entire target population with a hypothetical vaccine that is 100 percent effective.
The HAV vaccine would be administered to infants and young children, well before the disease usually occurs, so all cases and complications of the illness would be potentially vaccine preventable. Thus, the VPI estimates are identical to the disease burden estimates shown in Table G.2.
The concept of “infant mortality equivalence value” is used to standardize vaccine preventable illness scores, just as it is used to standardize disease burden values (see Chapter 4). Total vaccine preventable illness values for hepatitis A virus are calculated using estimates from Table G.2 and the two sets of IME values employed throughout this report. Using IME values based on a median of committee member perspectives, the total vaccine preventable illness value for hepatitis A virus is 181; with the age-neutral perspective the value is 176.