Reinfection

Reinfection is the occurrence of a second infection after recovery from an initial infection with the same pathogen or its close relative. It can involve the same microbial species but a different strain, serotype, or lineage. Reinfection is distinguished from relapse, where symptoms recur due to persistence or reactivation of the original infection, and, in some diseases, from recrudescence caused by residual organisms. Definitions and criteria vary by pathogen and by public health guidelines.

Mechanisms include renewed exposure to the pathogen after clearing the first infection, waning or incomplete immunity, and infection with a drifted or novel strain that escapes prior immune protection. In viral diseases such as SARS-CoV-2 and influenza, reinfections have been documented, particularly with immune-evasive variants. In malaria and some bacterial diseases, reinfection designations are used when a new infection follows full clearance; relapse or recrudescence refers to recurrence from existing parasites or bacteria.

Diagnosis is supported by epidemiology and laboratory testing, and in many cases requires genomic sequencing to show distinct strains or lineages. Serology alone cannot reliably distinguish reinfection from lingering immune responses. Time since primary infection, clinical presentation, and pathogen-specific criteria influence diagnosis, as does the threshold used by health authorities.

Prevention relies on reducing exposure and maintaining immunity through vaccination and boosters when available. Reinfections have important public health implications, including ongoing transmission and adjustments to vaccination strategies. Understanding reinfection informs risk assessment and disease control in endemic areas and during outbreaks.