Serial Interval and Generation Time

The generation time is the interval between the infection of a primary case and the infection of the secondary cases it causes, while the serial interval is the interval between the onset of symptoms in a primary case and in its secondary cases. Because infection times are usually unobserved, the serial interval is what surveillance can measure and is used as a practical proxy for the generation time; both quantities connect the timing of transmission to the estimation of how fast a disease spreads.

Definition

The generation time is the time between infection of a case and infection of the people it infects; the serial interval is the time between symptom onset in a case and symptom onset in the people it infects, used as an observable proxy for the generation time.

Scope

This entry distinguishes the generation time from the serial interval, explains how each relates to the incubation and infectious periods, and describes how these timing measures link epidemic growth rate to the reproduction number. It is a methodological reference topic, not clinical guidance.

Core questions

• How do the generation time and serial interval differ, and why does the distinction matter?
• How do these intervals relate to the incubation period and the infectious period?
• How is the serial-interval distribution used to estimate the reproduction number?
• Why is the generation time usually estimated rather than observed directly?

Key concepts

• Generation time (generation interval)
• Serial interval
• Incubation period
• Latent and infectious periods
• Serial-interval distribution
• Epidemic growth rate
• Estimation of the reproduction number from timing

Key theories

Generation interval and the growth-rate-to-R relationship

Wallinga and Lipsitch showed that the shape of the generation-interval distribution determines how an observed epidemic growth rate translates into a reproduction number, so timing measures are essential for interpreting transmissibility.

Mechanisms

Transmission timing is built from the natural history of infection: after infection there is a latent period before a host becomes infectious and an incubation period before symptoms appear, and the host transmits during its infectious period. The generation time measures infection-to-infection, while the serial interval measures onset-to-onset; the two coincide on average under some assumptions but differ in variance. Because infection times are rarely observed, analysts use the serial-interval distribution, together with the epidemic growth rate, to estimate the reproduction number, including methods that recover who-infected-whom probabilities or time-varying reproduction numbers from case-onset data.

Clinical relevance

Timing measures let analysts convert observed case curves into estimates of transmissibility and judge how quickly an outbreak is growing. They are reference quantities describing the transmission process and the generation of epidemiologic estimates, not a basis for individual diagnostic or treatment decisions.

Epidemiology

Estimated serial intervals and generation times vary by pathogen and are influenced by control measures that truncate the infectious period; reviews of incubation periods for respiratory viruses illustrate how natural-history timing differs across agents, and SARS analyses used onset data to estimate transmissibility. Reported intervals are therefore pathogen- and setting-specific.

History

The idea of the interval between successive cases is long-standing in epidemiology and was clarified by Fine's account distinguishing the serial interval from underlying transmission timing. In the 2000s, work by Wallinga and colleagues formalised the role of the generation interval in linking growth rates to reproduction numbers and developed practical estimation methods using onset data, with later software-based frameworks for time-varying reproduction numbers.

Debates

Can the serial interval safely stand in for the generation time?

The serial interval is observable but can differ from the generation interval in variance and can even be negative when pre-symptomatic transmission occurs, so using it as a proxy may bias reproduction-number estimates unless the difference is accounted for.

Key figures

• Jacco Wallinga
• Marc Lipsitch
• Paul Fine
• Anne Cori
• Justin Lessler

Seminal works

• wallinga-lipsitch-2007
• wallinga-teunis-2004
• fine-2003

Frequently asked questions

What is the difference between the serial interval and the generation time?

The generation time is measured between infection events, while the serial interval is measured between symptom-onset times; the serial interval is observable and is used as a proxy because infection times usually are not known.

Why are these intervals used to estimate the reproduction number?

The timing distribution between successive infections links the observed epidemic growth rate to the number of secondary cases per case, so it is needed to translate how fast cases rise into a reproduction number.