months of age. Other possible target populations include the elderly (Garvie and Gray, 1980), and older children with chronic cardiopulmonary disease (e.g., congenital heart disease, bronchopulmonary dysplasia, and asthma). RSV could be severe or fatal for children in this latter group at any age (MacDonald et al., 1982). Delivery of vaccine to pregnant or soon-to-be pregnant women may offer an alternative approach to immunizing young infants if the latter proves not to be practicable.
A vaccine conferring temporary immunity might be acceptable for the major target population because the period of highest vulnerability is so brief (the first year of life). This vaccine could be administered in the autumn in temperate climates where the disease is epidemic.
Severe RSV illness occurs in young infants, so vaccine prevention or amelioration of RSV illness will depend on the ability to develop a vaccine that can stimulate immunity at a very early age. Estimates below and predictions in Chapter 5 are predicated upon the assumption that this will be possible. A vaccine that produces immunity of relatively short duration may be acceptable for the reasons discussed above.
The feasibility of the alternative strategy—immunizing pregnant women—also needs to be investigated, especially if producing a vaccine immunogenic in young infants proves to be impossible.
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.
A large proportion of RSV illness in the first year of life occurs below the age of six months. Even assuming the development of a vaccine that is immunogenic in young infants, the vaccine would have to be delivered at a very early age, and in the case of the glycoprotein vaccine, only partial protection would be achieved before subsequent doses were administered. Because of these considerations, it is judged that only two-thirds of the burden of RSV in the first year of life would be vaccine preventable—even with the envisaged vaccines. All RSV illness in individuals 1–4 years of age is considered vaccine preventable.
Table N.2 shows a summary of the VPI for RSV.