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Vaccines work by training the immune system to recognise and rapidly respond to a specific pathogen without causing the disease itself. When a pathogen enters the body, the adaptive immune system generates antibodies and T cells specific to antigens — molecular markers on the pathogen's surface. This takes days to weeks. Vaccines introduce those antigens (or instructions to make them) in a harmless form: live-attenuated pathogens, killed pathogens, protein subunits, or — in mRNA vaccines like those for COVID-19 — genetic instructions for the body's own cells to produce the antigen. The immune system mounts a response and, crucially, forms long-lived memory B cells and T cells. On subsequent exposure to the real pathogen, these memory cells enable a response fast enough to neutralise the threat before it causes serious illness. Herd immunity occurs when a sufficiently high proportion of a population is immune — typically 70–95% depending on the pathogen's transmissibility (R₀) — so that chains of infection break and vulnerable individuals who cannot be vaccinated are indirectly protected.
answered by Omniscientia Team · 172 words · 18 Mar 2026