Sidemic

Sidemic is a term used in epidemiology to describe a secondary outbreak that accompanies a primary epidemic but tends to follow a distinct transmission pathway and population group. It is not a formal disease category but a conceptual construct to discuss concurrent waves with different dynamics.

Etymology and usage: The coinage combines side and epidemic, appearing in theoretical discussions and modeling studies in the 21st century. It is not uniformly defined and is used variably across disciplines, with some authors using it to highlight separate networks or modules within an outbreak.

Characteristics: A sidemic typically shows a lower peak and longer tail than the main epidemic, occurs in peripheral or marginalized communities, and may rely on different contact structures (for example, households versus workplaces). It can be temporally offset or intertwined with the primary epidemic and may persist after the major wave ends, acting as a reservoir for re-emergence or sparking secondary spread to core groups.

Modeling and detection: Sidemics are analyzed with coupled models (metapopulation, network-based, multi-strain). Detection requires disaggregated data by location, demographics, or network position, and time-series separation to distinguish from the main wave. Intervention strategies emphasize enhanced surveillance in secondary networks, targeted testing, and rapid adaptation of controls.

Examples and reception: The term is more common in teaching scenarios, hypothetical simulations, or scenario planning than in formal policy guidelines. Few peer-reviewed cases label an outbreak as a sidemic, and the concept remains debated in terms of definitional boundaries and practical utility.

See also: Epidemic, Endemic, Pandemic, Co-epidemic, Metapopulation, Network epidemiology.