postpartum period: the advantages and limitations of the different forms of chemoprophylaxis have been discussed elsewhere (Fischer et al., 1983). In general, chemoprophylaxis does not appear to be as practicable as vaccine control because of its complex logistical requirements for serological screening, culturing, and antibiotic administration.
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.
Over 50 percent of clinically significant neonatal GBS disease occurs in term infants. It is assumed that maternal immunization would provide passive immunity in the term neonate of sufficient duration to protect against both early- and late-onset disease. In the premature infant, however, even high levels of the appropriate (IgG) maternal antibody might not result in sufficient placental transport to confer protection. Therefore, only 90 percent of neonatal GBS disease is assumed in this exercise to be potentially preventable by maternal immunization. Theoretically, it also should be possible to prevent 100 percent of maternal disease caused by GBS with an ideal vaccine that decreased or eliminated carriage of GBS. Table P.5 shows a summary of VPI for GBS.
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). Vaccine preventable illness values for GBS are calculated using estimates from Table P.5 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 GBS is 3,570; with the age-neutral perspective the value is 3,528.
To calculate the possible reduction in morbidity and mortality (the maximum potential health benefits) that could be produced by the GBS vaccine candidate, the total vaccine preventable illness value for each IME perspective is multiplied by the predicted efficacy of the