Vaccine contraindications may prevent certain individuals from being vaccinated. Ī portion of those vaccinated may not develop long-term immunity. Individuals who are immunodeficient due to HIV/AIDS, lymphoma, leukemia, bone marrow cancer, an impaired spleen, chemotherapy, or radiotherapy may have lost any immunity that they previously had and vaccines may not be of any use for them because of their immunodeficiency.

Newborn infants are too young to receive many vaccines, either for safety reasons or because passive immunity renders the vaccine ineffective.

Some individuals either cannot develop immunity after vaccination or for medical reasons cannot be vaccinated. Herd immunity protects vulnerable communities. Effects Protection of those without immunity Herd immunity created via vaccination has contributed to the reduction of many diseases. Once the herd immunity has been reached, disease gradually disappears from a population and may result in eradication or permanent reduction of infections to zero if achieved worldwide. It occurs when a sufficient percentage of a population has become immune to an infection, whether through previous infections or vaccination, thereby reducing the likelihood of infection for individuals who lack immunity.

Herd immunity (also called herd effect, community immunity, population immunity, or mass immunity) is a form of indirect protection that applies only to contagious diseases. In the first two examples, most healthy unimmunized people become infected, whereas in the bottom example only one fourth of the healthy unimmunized people become infected. In the bottom box, a large proportion of the population have been immunized this prevents the illness from spreading significantly, including to unimmunized people. The middle box shows a population where a small number have been immunized (shown in yellow) those not immunized become infected while those immunized do not. The top box shows an outbreak in a community in which a few people are infected (shown in red) and the rest are healthy but unimmunized (shown in blue) the illness spreads freely through the population.

Herdprotect