Key Terms

The following definitions are provided for quick reference. Full context for each appears in the relevant section of the guide.

Adaptive Immunity

The slower-engaging, highly specific arm of the immune system, responsible for antibodies and memory cells tailored to particular pathogens. What vaccines are designed to train.

Adjuvant

An ingredient added to some vaccines specifically to boost the strength of the immune response they generate — used in some modern vaccines to compensate for weaker vaccine responsiveness in older adults.

Herd Immunity

The population-level effect in which sufficient vaccination or prior-infection coverage limits a pathogen's ability to spread, indirectly protecting those who cannot be vaccinated themselves.

Immunosenescence

The well-documented decline and dysregulation of immune function with age, contributing to both higher infection risk and reduced vaccine responsiveness in older adults.

Inactivated Vaccine

A vaccine using a killed version of a pathogen, incapable of causing disease but generally requiring booster doses since the resulting immune response tends to fade faster than with live-attenuated vaccines.

Innate Immunity

The fast, non-specific first line of immune defence — skin, mucus, inflammation, and general-purpose immune cells — that responds within minutes to hours without "remembering" specific past threats.

Live-Attenuated Vaccine

A vaccine using a weakened but still-replicating version of a pathogen, typically producing strong, long-lasting immunity closely mimicking natural infection.

mRNA Vaccine

A vaccine delivering genetic instructions that prompt the recipient's own cells to temporarily produce a harmless pathogen fragment, training immune recognition. The mRNA degrades within days and does not alter DNA.

Subunit / Recombinant Vaccine

A vaccine using only a specific protein fragment of a pathogen, sufficient to train immune recognition without using any component capable of causing illness.