Breakthrough infection

A breakthrough infection occurs despite a preventive measure intended to stop infection, especially vaccination or antimicrobial prophylaxis. It can arise when protection is incomplete, wanes, or is overcome by exposure.

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Vaccine effectiveness: The reduction in disease risk among vaccinated people compared with an appropriate unvaccinated comparison group. Effectiveness measures how much vaccination lowers risk without implying that every infection is prevented.
COVID-19 breakthrough infection: A SARS-CoV-2 infection occurring in a person who has received a COVID-19 vaccine. These cases brought the term into widespread use and exposed differences between infection and severe disease protection.
Post-exposure prophylaxis: Medication or other intervention given after a possible pathogen exposure to prevent infection or disease. Infection after this intervention is a breakthrough despite prevention begun after exposure.
Vaccine failure: The occurrence of disease in a vaccinated person because vaccination did not provide sufficient protection. This broader outcome can include disease without a confirmed infection or infection without symptoms.
Waning immunity: The gradual decline of immune protection over time after infection or vaccination. Falling protection can make infection more likely long after an initially effective vaccine dose.
Breakthrough varicella: A mild varicella infection occurring more than 42 days after vaccination against chickenpox. This established a well-defined vaccine-specific example, often milder than unvaccinated chickenpox.
Pre-exposure prophylaxis: Preventive medication taken before anticipated exposure to reduce the risk of acquiring an infection. Breakthrough infections can occur during prophylaxis and prompt assessment of adherence or resistance.
Vaccine escape: Reduced vaccine protection caused by pathogen changes that evade vaccine-induced immune recognition. Escape identifies a pathogen-side cause, whereas breakthrough names the infection outcome.
Immune escape: The ability of a pathogen to evade recognition or control by host immune defenses. Pathogen changes can reduce recognition by antibodies or other vaccine-induced defenses.
Breakthrough malaria: Malaria occurring despite use of preventive measures such as antimalarial drugs or insecticide-treated nets. Malaria illustrates that prophylaxis reduces risk without guaranteeing protection from infection.