The need for early life immunizationNeonatal or early infant immunizations are required to give protection from diseases caused by pathogens to which exposure occurs in early life. As an example, the median age of infants with a respiratory syncytial virus (RSV) infection severe enough to require hospitalization is below 3 months of age worldwide. A significant proportion of severe early infections are due either to encapsulated bacteria such as Haemophilus influenzae, Streptococcus pneumoniae, or Neisseria meningitidis or to intracellular agents such as viruses [RSV, measles virus (MV), rotavirus, herpes simplex viruses, cytomegalovirus], certain bacteria (Listeria monocytogenes, salmonella, mycobacteria) or other microorganisms (Chlamydiae, Ureaplasma). This enhanced susceptibility of newborns and infants to specific infectious diseases is due to limitations of both their innate and specific immune system.
Immune responses in early life differ from adult responsesThe impaired function of monocytes/macrophages in newborns has been associated with reduced expression of major histocompatibility complex (MHC) class II and costimulatory molecules, decreased antigen-processing capacity and altered cytokine production. Although little is known about the development of human dendritic cells, their maturation appears to be slow and their function as antigen-presenting cells (APC) is rather poor at birth [10], thus probably contributing to the limitations of the induction of specific immune responses. Although natural killer (NK) cells are present in significant numbers at birth, their functional activity [reflected by interferon-y (IFN-y) production and NK cell-mediated lysis] also remains low for several weeks. It is not yet known precisely which part of these limitations reflects the intrinsic immaturity of infant NK cells or their impaired activation by cytokines or factors that prevail in the Correspondence to." C.-A. Siegrist