the first six months of life. The aim would be to prevent as much PIV-3 disease as possible, and also to reduce PIV-1 and PIV-2 infections, which usually occur later.
High levels of passively acquired maternal antibody appear to play a role in protecting infants against parainfluenza viruses during the first year of life (Glezen et al., in press), so a vaccine administered to pregnant women also might be a possibility, identification of an appropriate PIV vaccine candidate for pregnant women will require more research on the nature of antibodies induced by PIV infection and the extent to which they cross the placenta.
Illness caused by parainfluenza viruses types 1 and 2 occurs predominantly after six months of age, so an opportunity exists to deliver the vaccine prior to the peak of illness. The peak of illness for parainfluenza virus type 3 occurs earlier, however, before the full required number of doses could be delivered. Thus, a lower proportion of these illnesses could be averted. While reinfection does occur, indicating that natural immunity is not fully protective, it is probable that a vaccine could, at a minimum, avert the more severe 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 vaccine envisaged by the committee would require two early doses and probably additional doses to boost or maintain immunity. While the major portion of illness caused by PIV-1 and PIV-2 occurs after six months, the pattern of PIV-3 illness is similar to that of RSV, involving a considerable amount of severe illness under six months of age. The first dose of vaccine probably could be administered at about two months, with a second dose two to three months later. Vaccinees would be only partially protected during this period; hence, only about 80 percent of the cases of illness occurring under one year of age are judged to be vaccine preventable. All cases occurring in older age groups (1–4 years) are judged to be vaccine preventable. Since deaths are thought to be due predominantly to PIV-3 infections in young infants, only about 50 percent of deaths are considered vaccine preventable.
Natural immunity is not fully protective or is short lived so reinfection does occur, although it is milder. Hence, the vaccine is predicted to reduce the severity of illness rather than totally prevent cases of the disease. Reductions in the severity of cases in Category