Table 3.

Factors Impacting Vaccine Failure

Pathogen Factors	• Antigenic drift/distance between vaccine and circulating strains • Co-infections—interference • Viral decoy mechanisms • Viral immune evasion mechanisms
Host Factors	• Immunocompromised • Co-morbidities • Age/immunosenescence • Obesity • Genetic restriction • Medications • Negative interference • Role of humoral vs. cellular immunity • Waning immunity • Pre-existing infection • Immunologic interference
Vaccine Factors	• Improper administration • Differences in immunogenicity (adjuvanted vs. non-adjuvanted) • Dosage and number of doses • Time between vaccination and development of immunity • Egg-passage-induced antigenic changes in vaccine • Handling • Temperature inactivation • Vaccine-vaccine interference
Study/Study Design Factors	• Geography • Confounding biases • Efficacy vs. effectiveness • Prevalence of infection • Study design • Specificity of study outcomes • Year of study • Laboratory assays used • Incubation period vs. timing of immunization

Table Adapted from CIOMS/WHO Working Group on Vaccine Pharmacovigilance. Definition and Application of Terms for Vaccine Pharmacovigilance. http://www.who.int/vaccine_safety/initiative/tools/CIOMS_report_WG_vaccine.pdf. 2011. Date accessed: November 28, 2017.”