The incidence of coccidioidomycosis rises rapidly in persons over the age of 15. Fraser and coworkers (1979) have reported that the incidence of hospitalization for coccidioidomycosis begins to rise at age 15 and shows a steady increase until age 25, after which the overall incidence remains fairly stable. Immunization efforts probably would be directed at 15-year-olds, although initially older persons would be vaccinated too. Younger target populations might be considered in some cases where substantial exposure occurs at an earlier age, e.g., some native American populations.
The natural course of most coccidioidomycosis infections suggests that the disease is suitable for vaccine control. An effective vaccine would be expected to induce satisfactory immunity, because primary natural infection usually provides resistance to exogenous reinfection.
Alternatives to vaccine prevention are unsatisfactory: avoidance of infection in endemic areas is virtually impossible. The disease could be prevented if it were possible to decontaminate infected soil (because infection is transmitted primarily by inhaled arthroconidia), but it is not. Also, the therapeutic alternatives for treatment of coccidioidomycosis are limited. Amphotericin B is the standard drug for chemotherapy and has been used for more than 20 years in pulmonary coccidioidomycosis. In practice, however, it appears that while amphotericin has a fungicidal activity in vitro, it is only fungistatic in concentrations that are clinically attainable in body fluids (Stevens, 1980). It is also highly toxic, so its benefits must be weighed carefully against its toxicity.
While the disease burden in terms of number of cases is relatively small compared to that associated with many other infectious diseases, treatment costs are significant (see below). Effective vaccination strategies probably could be designed for some subsets of the population at risk (e.g., some occupational groups), and these strategies might significantly reduce costs associated with the disease. It is important to note, however, that some of the groups at highest risk, especially migrant workers, would be difficult to reach.
Defining the target population is the first step in calculating the possible reduction in morbidity and mortality 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.
On the basis of the suggested target population, described above, all illness occurring in persons older than age 15 is considered