Definition of Vaccine, according to W.H.O., WHO.int
Pre 2015: "Injection of a killed or weakened infectious organism in order to prevent the disease."
2015-2021: "The act of introducing a vaccine into the body to produce immunity to a specific disease."
September 2021: "The act of introducing a vaccine into the body to produce protection from a specific disease."
Definition of influenza pandemic, according to W.H.O., WHO.int
Pre 2009: "An influenza pandemic occurs when a new influenza virus appears against which the human population has no immunity, resulting in epidemics worldwide with enormous numbers of deaths and illness."
May 2009: "A disease epidemic which occurs when there are more cases of that disease than normal."
Definition of Herd Immunity, according to W.H.O., WHO.int
November 2020: "Herd immunity is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection."
October 2021: "WHO supports achieving 'herd immunity' through vaccination, not by allowing a disease to spread through any segment of the population, as this would result in unnecessary cases and deaths."
