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Vaccines for the 21st Century: A Tool for Decisionmaking
The feasibility of immunizing pregnant women for the prevention of several infectious diseases whose contemporary burdens in young infants are substantial has been extensively investigated. The vaccines studied include those consisting of antigens from bacterial pathogens (GBS [Baker et al., 1988], Hib [Mulholland et al., 1996], Neisseria meningitidis groups A and C, Streptococcus pneumoniae [McCormick et al., 1980; O’Dempsey et al., 1996] and viral pathogens (HBV, influenza virus, rabies, RSV, yellow fever virus), and others have been proposed for study (CMV, herpes simplex virus, human immunodeficiency virus, rotavirus). Immunization of pregnant women is an approach that can avoid the obstacles presented by the immunologic immaturity of the neonate, has the potential to defer the need for active immunization of the infant or to decrease the number of doses needed to achieve protection during the first year of life, and may prevent the transmission of infection from the mother to the neonate. Despite considerable scientific data underscoring the feasibility, safety, and cost-effectiveness of this approach, progress in the United States has been slow or wanting. Obstacles to this approach are said to involve ethical, legal, and sociological concerns, but issues concerning the liability of vaccine manufacturers dominate the others (Insel et al., 1994; Linder and Ohel, 1994). Scientific evidence pales when concern over the liability of vaccine manufacturers arises, and vaccine manufacturers appeal to the government for indemnification before they pursue studies of existing vaccines or the development of additional reagents appropriate for immunizing pregnant women.
The committee reviewed immunization of pregnant women as a vaccination strategy in some detail, believing it to be scientifically valid for use in the United States for preventing several infectious diseases in young infants. However, this approach deserves special consideration because of the obstacles mentioned above. Thus, it is appropriate to underscore the rationale for this approach. Alternatives are discussed and critiqued in Chapter 7.
Passive immunization of the neonate and young infant depends on selective placental transfer of plasma proteins, and the transfer of immunoglobulins is limited to immunoglobulin G (IgG). Passage of antibodies is both passive (directly proportional to the maternal serum IgG concentration) and active (binding of IgG to Fc receptors, followed by receptor-mediated endocytosis). The latter accounts for the preferential transport of IgG 1 and IgG3, which have greater affinities than IgG2 for binding to Fc receptors. Passage of antibodies begins at 8 weeks of gestation, but the level remains low until about 20 weeks. Neonatal cord serum IgG levels correlate with gestational age. At 32 weeks of gestation cord serum has an IgG level approximately half of that at 40 weeks, when the levels are equal to or somewhat greater than those in maternal serum. Maternal IgG has a half-life of 3 to 4 weeks, but since the duration of passive antibody protection is dictated by the actual level at birth, protection may last for 3 to 6 months.
The immune response of pregnant women is similar to that of nonpregnant women (Halsey and Klein, 1990), but there is a delay between the time of immunization and the time of fetal acquisition of maternal antibodies. This is the interval required for the mother to generate an IgG antibody response and for the