Vaccination part 10
Discussions regarding the vaccination of infants often refer to the infant’s immune system as “immature;” this needs some clarification, as the issue is not an inadequacy, but a differing posture. “Developing” might be a more appropriate description. Fundamentally, the immune system of infants is engaged in processes that differ from that in older individuals.
An essential aspect of acquired/adaptive immunity is the learning of “self,” so that immune responses are not directed at host tissues (including the essential microbial biome). To this end, the vast random diversity (10^15) of developing B- & T- lymphocyte clones needs to be culled in a process of clonal deletion, eliminating those clones directed to discovered self-epitopes. This is an essential chore of the infant’s adaptive immune system that needs to be accomplished before being challenged to counter non-self threats. I wouldn’t describe this so much as “immaturity,” as an essential stage of development.
During this developmental stage, the infant’s immune system takes on a non-inflammatory stance, & is dependent on innate & passive immune mechanisms to protect the organism from infectious disease.
Transplacental transfer of Maternal IgG and T cells, and transfer of maternal IgA, IgG, IgM, IgD, & T lymphocytes via breastmilk, provide passive immunity essentially equivalent to that possessed or acquired by the mother, for at least the first 6 months of post-uterine life, and extending well beyond this in breastfed infants.
Transplacental transfer of passive immunity begins as early as 13 weeks gestation, but is most active in the late 3rd trimester, leaving infants born prematurely with less protection than those carried to term. Transfer of immune factors from breastfeeding is most effective in colostrum over the initial 24 hrs following birth, but continues throughout breastfeeding, and includes immune factors (most significantly IgA) directed to current disease exposure of the maternal/infant pair.
Vaccination in this early period of life is often unsuccessful. The measles vaccine, e.g., rarely results in an antibody & T-cell response in children under 18 months of age. The timing of the first MMR vaccination has been pushed back from 18 to 12–15 months due to the increased risk of measles in infants in recent years (since widespread MMR vaccination has altered the demographics of measles), but its efficacy in this younger age group is questionable.
Adjuvants (aluminum salts, squalene derivatives, other highly inflammatory agents) are often included in vaccines to enhance the innate inflammatory response to the vaccine at the injection site, enhancing uptake of the active vaccine component(s) by antigen-presenting cells. These may overcome the non-inflammatory stance of the infant’s immune system, but the wisdom of this might be questioned. We don’t yet fully appreciate the potential consequences of inciting inflammation and forcing an adaptive immune response in the developing immune system.
It’s been observed that maternally conferred passive immunity might interfere with an immune response to vaccination, which frankly sounds like an ass-backwards concern to me. The provision of maternally-acquired passive immunity to the neonate via transplacental transfer of IgG & T-cells and transfer of IgA, IgG, IgM, IgD, & T lymphocytes in colostrum & breastmilk provides a brilliant solution to the necessary non-inflammatory stance of the developing immune system of the infant.
Some vaccination protocols take advantage of maternally conferred passive immunity. The Tdap (tetanus, diphtheria, acellular pertussis) vaccine is currently recommended in pregnancy at 27-36 weeks gestation, and appears to reduce the risk of pertussis in infants under 2 months of age by enhancing maternally provided passive immunity. The Abrysvo® RSV vaccine is currently (as of May 2023) approved for use in women between 32 and 36 weeks gestation if this is between September & January, to target births that would expose an infant to the typical October-April RSV season in their first year of life, both of these providing vaccine-induced immunity via maternally conferred factors to infants too young to respond to vaccination directly.
The short- & long-term consequences of inciting inflammation & inviting an adaptive immune response in infants is not as yet well appreciated. Nevertheless, a number of vaccinations are recommended by public health agencies in this early period of life:
Although there is speculation that this might potentially contribute to the development of autoimmune disease or other long-term consequences, truth is we’re very much in the dark, while struggling to find means for the prevention of some potentially serious infectious diseases.