Herd immunity
Herd immunity is indirect protection that arises when enough people in a population are immune to an infectious disease to limit its spread.
Basic reproduction number: The average number of secondary infections caused by one infected person in a wholly susceptible population. It helps estimate how much immunity can prevent an outbreak from growing.
Vaccination: The administration of a vaccine to stimulate protective immune responses against an infectious disease. Vaccination raises population immunity without requiring widespread infection.
Individual immunity: An individual's reduced susceptibility to infection or disease after immune defenses develop. Herd immunity is a population effect, not a guarantee that any particular immune person is protected.
Vaccine hesitancy: Delay in acceptance or refusal of vaccines despite their availability. Clusters of low uptake can leave transmission pathways even when average coverage appears high.
Antigenic drift: The gradual accumulation of mutations in a pathogen's antigens that can alter recognition by immune defenses. Immune escape can weaken protection against transmission and shift the immunity needed to limit spread.
Effective reproduction number: The average number of secondary infections caused by an infected person under current conditions. Herd immunity is reached when transmission falls below replacement, with this number below one.
Measles: A highly contagious viral disease that can cause fever, rash, and serious complications. Its high transmissibility makes high and sustained vaccine coverage essential to limiting outbreaks.
Natural infection: Infection acquired through exposure to a pathogen rather than through vaccination. Infection can increase immunity, but pursuing it carries illness, death, and transmission costs.
Immunocompromised person: A person whose immune defenses are weakened by disease, treatment, or another cause. Indirect protection can reduce exposure for people who respond poorly to vaccines.
Waning immunity: The gradual decline of immune protection over time after infection or vaccination. As protection fades, previously immune people can re-enter the susceptible pool.