Breakthrough infection

A breakthrough infection 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 (currently only viruses).^[1] Breakthrough infections have been identified in individuals immunized against a variety of diseases including mumps, varicella (Chickenpox), influenza, and COVID-19.^[2][3][4] The characteristics of the breakthrough infection 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.^[5]

Causes of breakthrough infections 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.^[6] 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.^[7] Accordingly, herd immunity reduces the number of breakthrough infections in a population.^[8]