Source Identification and Transmission Chains

Identifying the source of an outbreak and reconstructing how infection passed from one person or vehicle to the next is the analytic heart of an investigation. Source identification asks where the agent came from, while transmission-chain analysis asks how it spread, linking cases into the sequence of who infected whom and revealing the route, the index case, and any amplifying events.

Definition

Source identification is the determination of the origin, vehicle, or reservoir of an outbreak, and transmission-chain reconstruction is the linking of cases into the inferred sequence of person-to-person or source-to-person transmission, using epidemiologic, temporal, and increasingly genomic evidence.

Scope

This topic covers the reasoning and tools used to attribute an outbreak to a source and to map its chains of transmission: epidemic-curve interpretation, the distinction between point-source and propagated patterns, the concepts of index and primary cases, the reproduction number, super-spreading, and the increasing use of pathogen genome sequencing alongside epidemiologic linkage. It is conceptual and methodological, not a protocol for any specific pathogen or response.

Core questions

• Where did the outbreak agent originate, and through what vehicle or reservoir did exposure occur?
• Does the epidemic curve indicate a point-source, continuous, or propagated pattern?
• Who is the index or primary case, and who infected whom?
• How can pathogen genome sequences be combined with epidemiologic data to confirm linkage?

Key concepts

• Point-source versus propagated outbreak
• Index case and primary case
• Mode and route of transmission
• Basic and effective reproduction number
• Super-spreading and individual variation
• Serial interval and generation time
• Genomic epidemiology and transmission inference

Mechanisms

The shape of the epidemic curve is the first clue to source: a sharp single peak suggests a common point source, a sustained plateau a continuous source, and successive waves spaced by the serial interval a propagated, person-to-person spread. Investigators trace exposures backward to a common vehicle or reservoir and identify index and primary cases. Transmission chains are then reconstructed by linking cases through timing, place, and contact, and increasingly by comparing pathogen genome sequences, since closely related sequences support a direct epidemiologic link. The reproduction number summarises how many secondary cases each case generates on average, while super-spreading reflects the large individual variation that can drive explosive transmission.

Clinical relevance

Establishing the source tells responders what to remove, recall, or close, and mapping transmission chains shows where to direct contact tracing and protective measures. For clinicians, recognising that a case may be one link in a chain underlines the value of detailed exposure histories and timely reporting. This entry describes how source and spread are inferred and is not guidance for treating an individual patient.

Epidemiology

Person-to-person transmission was demonstrated early in the COVID-19 pandemic through a familial cluster in which infection passed between household members, illustrating chain reconstruction at the smallest scale. At the population scale, analysis of SARS epidemic curves estimated the reproduction number and the effect of control measures, and studies of super-spreading showed that a minority of cases can generate most secondary infections; pathogen genome sequencing now routinely complements such epidemiologic linkage.

History

John Snow's nineteenth-century attribution of a cholera outbreak to the Broad Street pump is the archetypal source identification by mapping cases to a common exposure. Twentieth-century field epidemiology formalised epidemic-curve interpretation and the point-source versus propagated distinction, while the twenty-first century added quantitative transmission modelling, recognition of super-spreading from SARS, and whole-genome sequencing, which together sharpened the reconstruction of transmission chains.

Debates

Can genome sequences alone establish who infected whom?

Pathogen genomic data greatly strengthen transmission inference, but limited within-host diversity, shared sources, and sampling gaps mean that sequences usually constrain rather than definitively prove a direct link, so genomic and epidemiologic evidence are best combined.

Key figures

• Jacco Wallinga
• James Lloyd-Smith
• Marc Lipsitch
• Yonatan Grad

Seminal works

• wallinga-teunis-2004
• lloyd-smith-2005
• grad-lipsitch-2014

Frequently asked questions

What does the shape of an epidemic curve tell investigators about the source?

A single sharp peak suggests a common point source, a prolonged plateau suggests a continuous source, and a series of progressively larger waves spaced by the disease's serial interval suggests propagated person-to-person transmission.

What is super-spreading?

Super-spreading describes the large variation between individuals in how many secondary infections they cause; a small number of cases or events can account for a disproportionate share of transmission, which strongly shapes how outbreaks grow and how they can be controlled.