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respiratory infections also begins to decline. The reasons for these shifts are complex, in part because of the effects of malnutrition, environmental pollution, and other risk factors.

Even with complete mortality statistics it is difficult to establish the role of the parainfluenza viruses, because numerous other pathogens also cause respiratory infections in children. Respiratory syncytial virus (RSV) and adenoviruses (particularly in the Far East) produce similar symptoms. In addition, bacterial superinfections may occur and can contribute to mortality.

The disease burden estimates for parainfluenza virus infection are shown in Table D-9.1, and their derivations are described in Appendix B. It should be emphasized that these estimates are uncertain because of the lack of data on parainfluenza in developing countries. Acute lower respiratory tract illness from parainfluenza virus infection may eventually contribute to chronic obstructive pulmonary disease (Glezen, 1984). However, no attempt has been made to include such a contribution in chronic morbidity estimates. This aspect of the disease burden of the parainfluenza viruses requires periodic reevaluation.

PROBABLE VACCINE TARGET POPULATION

The most severe illnesses caused by parainfluenza virus infections occur in the first years of life. Hence, the target population would be infants at the earliest feasible age. The simplest design for the use of a PIV vaccine would be to administer it during the first 6 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 (at least in the United States). It is likely that a subunit vaccine could be incorporated into the World Health Organization Expanded Program on Immunization (WHO-EPI) delivery schedules, possibly in combination with other vaccines that are delivered at an early age, e.g., DTP.

Because 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., 1984), a vaccine administered to pregnant women also might be effective. 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.

Vaccine Preventable Illness*

The vaccine envisaged by the committee would require two early doses and probably additional doses to boost or maintain immunity. In

*  

Vaccine preventable illness is defined as that portion of the disease burden that could be prevented by immunization of the entire target population (at the anticipated age of administration) with a hypothetical vaccine that is 100 percent effective (see Chapter 7).



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