“The frequent statement that high levels of vaccination prevent disease outbreaks is not accurate as infectious diseases do in fact occur even in fully vaccinated populations as well as individuals. The likely reason for this is that vaccines primarily stimulate humoral immunity (antibody-based or Th2 responses) while they have little or no effect on cellular immunity (cytotoxic T-cells, Th1 responses), which is absolutely crucial for protection against viral as well as some bacterial pathogens.
This may be the reason why vaccine-induced immunities are transient, requiring booster shots, while naturally acquired immunity conferred by the cellular immune system in the absence of vaccination tends to be permanent.
Taken together, these observations may explain why outbreaks of allegedly vaccine-preventable diseases do occur in fully vaccinated populations and why immunity (or its absence) cannot be reliably determined on the basis of serologic determination (measure of antibody levels), which is the most common measure of vaccine efficacy in clinical trials.
It should be noted that there is an instance where vaccinations could induce T-cell (Th1) responses and this is true in the case of repetitive immunizations with the same antigen (i.e., closely spaced ‘booster shoots’) however, the induction of such immune responses is deleterious as demonstrated by Tsumiyama et al who showed that CD4+ T cells from repeatedly immunized mice acquire the ability to induce autoantibodies which results in autoimmune tissue injury akin to that seen in human autoimmune diseases.
As previously mentioned, from these experiments Tsumiyama et al. concluded that systemic autoimmunity appears to be the inevitable consequence of over-stimulating the host’s immune ‘system’ by repeated immunization with antigens.”
— Lucija Tomljenovic, PhD, biochemist with the Neural Dynamics Research Group at the University of British Columbia, Canada