## 4 Interactions between Vaccine Administration and Gut Microbiome Status
### Gut Dysbiosis and Its Influence on Vaccine Efficacy in Poultry  
As detailed in the previous section, human case studies have evaluated causative agents of reduced vaccine immunogenicity over the past two decades and discovered that chronic enteric inflammation or extensive antibiotic use may lead to vaccine hyporesponsiveness in children. Human health is diversely influenced by a multitude of factors (socio-economic background, diet, region, age), and comparisons between discoveries in human health and those in research animal models (including poultry) must be made with ample trepidation. Conversely, understanding factors that influence vaccineefficacy in human case studies could help evaluate causes of vaccine hyporesponsiveness in animal health management. When discussing vaccine hyporesponsiveness in poultry, investigations regarding the relationship between gut microbiome status and vaccine efficacy have been very limited (Table 1). As previously noted, commercial poultry houses have high microbial loads [10; 11] that could disrupt the microbiome of young chicks with undeveloped immune systems [16; 33; 64]. Only in recent years have exploratory studies evaluated the effects of antibiotic administration in broiler flocks on the efficacy of a variety of vaccines [65; 66] or inflammation when challenged with live pathogens [67]. Several studies have also investigated the influence of probiotic supplementation on the immunological and gut microbiome states of poultry flocks [68; 69; 70]. Antibiotic growth promoters were a common management practice in commercial poultry production [71]. Although the poultry industry in the U.S. and E.U. are moving away from antibiotic applications used solely for growth promotion [72], recent studies have evaluated the effects of prolonged antibiotic applications on vaccine responses in poultry flocks. [PERSON] et al. [66] observed that specific pathogen-free (SPF) layer chickens treated with an antibiotic cocktail in drinking water for 35 days and vaccinated with an avian influenza vaccine had greater IFN-\(\gamma\) expression by splenocytes along with lower levels of _Firmicutes_ and _Bacteroidetes_ phyla in the ceca when compared to birds receiving the same antibiotic cocktail but treated with fecal microbial transplantation or a probiotic formulation during vaccine administration. _Firmicutes_ and _Bacteroidetes_ are both major contributors to the gastrointestinal tract microbiota and are generally associated with commensal microbes that secrete lactic acid (like _Lactobacillus_ spp.) and microbes essential for complex polysaccharide degradation and bile acid deconjugation, respectively [73]. Similar immunological responses were observed in White Leghorn chickens that were intramuscularly vaccinated with an inactivated H9N2 influenza virus and supplemented with a _Lactobacillus_ probiotic cocktail, i.e., greater IFN-\(\gamma\) mRNA expression in splenocytes as well as increased hemagglutination inhibition titers in serum [74]. In mouse models, opposing results have been observed with antibiotic-induced dysbiosis in the gut. Following an aerosol _Mycobacterium tuberculosis_ challenge, mice subcutaneously vaccinated against _M. tuberculosis_ and administered an antibiotic cocktail exhibited lower CD4\({}^{+}\) and CD8\({}^{+}\) T cell proliferation in the lung mucosa following a secondary immunization [58]. Additionally, C57 BL/6 mice reared in specific pathogen-free conditions that were administered oral antibiotic treatments found that the type of antibiotic cocktail can influence the antibody isotype response to a variety of vaccine antigens without affecting the expression of lymphocyte chemotaxis receptors in splenic and mesenteric tissues [75]. More specifically, broad-spectrum antibiotic administration increased serum and secretory IgA levels among the vaccine antigen treatments but did not induce IgG1 responses in the serum, indicating regulatory effects of antibody responses by the microbiome that are not yet understood [75].