Breakthrough infection

A breakthrough infection is a case of illness in which a vaccinated person becomes infected because the vaccine did not give complete immunity against the pathogen, which so far means viruses. It has been documented after immunization against mumps, chickenpox, influenza and COVID-19, and its character depends on the virus. The infection in a vaccinated person is often milder and shorter than natural infection would be. Causes include biological factors in the recipient, improper administration or storage of the vaccine, viral mutation and blocked antibody formation. Because vaccines are rarely fully effective, such cases are expected: a 2021 study found the common flu vaccine protected 58 percent of recipients, and the measles vaccine fails to protect about 2 percent of vaccinated children. Herd immunity lowers the number of breakthrough infections in a population.