Fig. 2. Factors with the potential to influence vaccine immunogenicity and/or efficacy.

A range of intrinsic host factors (such as age, sex, genetics and comorbidities) and extrinsic factors (such as perinatal, nutritional, environmental and behavioural factors) have been suggested to influence vaccine immunogenicity and/or efficacy (reviewed in detail in ref.⁴). The influence of these factors on vaccine immunogenicity is likely mediated indirectly via the effects of these factors on baseline immunity and/or the composition of the microbiota. Vaccine immunogenicity is also, of course, dependent on vaccine-intrinsic factors such as the adjuvant used, and vaccine efficacy may be influenced by factors other than vaccine immunogenicity such as the degree of match between the vaccine and the strains circulating at the time.