A<unk>  is a case of illness in which a vaccinated individual becomes infected with the illness, because the vaccine has failed to provide complete immunity against the pathogen (viruses only currently).<unk> s have been identified in individuals immunized against a variety of diseases including mumps, varicella (Chicken Pox), influenza, and COVID-19. The characteristics of the<unk>  are dependent on the virus itself. Often, infection of the vaccinated individual results in milder symptoms and shorter duration than if the infection were contracted naturally. Causes of<unk> s include biological factors in the recipient, improper administration or storage of vaccines, mutations in viruses, blocking antibody formation, and other factors. For these reasons, vaccines are rarely 100% effective. A 2021 study found the common flu vaccine provided immunity to the flu in 58% of recipients. The measles vaccine fails to provide immunity to 2% of children that receive the vaccine. However, if herd immunity exists, it typically prevents individuals who are ineffectively vaccinated from contracting the disease. Accordingly, herd immunity reduces the number of<unk> s in a population.
Breakthrough infection
Spillover infection
Subclinical infection
Epizootic
Window period
Pandemic
Exacerbation
Disease cluster