Table 1.

Definitions

Immunogenicity	Ability of a vaccine to induce an adaptive immune response (humoral and/or cell-mediated)
Reactogenicity	Tendency of a vaccine to produce expected, short-term adverse reactions such as pain at the injection site, fever, or fatigue
Adjuvant	Substance used in a vaccine to enhance immune response against the antigen. Common adjuvants include aluminum-based (HAV, HBV except Heplisav-B, 9vHPV, PPSV23), oil-in-water emulsion MF59 (influenza), Toll-like receptor (TLR) 4-agonist AS01 (Shingrix, RSV Arexvy), TLR9-agonist CpG 1018 (Heplisav-B)
Seroprotection	Antibody level above a pre-defined cut-off that indicates disease immunity
Seroconversion	A significant increase in antibody levels, often defined as a fourfold rise, or a change from a seronegative to seropositive state, that indicates an immune response to a vaccination or infection
Vaccine effectiveness	Percentage reduction in the rate of disease among vaccinated compared to unvaccinated individuals in real-world settings
Vaccine efficacy	Percentage reduction in the rate of disease among vaccinated compared to unvaccinated individuals in controlled clinical trial settings
Vaccine hesitancy	Delayed- or non-vaccination despite vaccine availability; influenced by factors like mistrust, misinformation, and concerns about safety or efficacy