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Generation time and serial interval
Which term describes the interval between a person acquiring an infection and reaching maximum infectivity, and which describes the gap between symptom onset in a primary case and in the secondary case they infect?
- A. Incubation period; latent period
- correctB. Generation time; serial interval
- C. Serial interval; generation time
- D. Latent period; communicable period
Explanation
Key fact
Generation time is measured from infection; serial interval from symptoms. One is about the organism, the other about what you can observe.
Generation time, then serial interval. The serial interval is what outbreak investigators can actually measure.
Why
Generation time runs from acquiring infection to peak infectivity.
It cannot be observed directly, because infection is not seen.
Serial interval runs from symptom onset in one case to onset in the next.
That is observable, so it is used as a proxy in outbreak work.
Latent period is from infection to becoming infectious to others.
The intervals of an infection, on one timeline
Why not the others
- A. Incubation period; latent period — names two real intervals but neither answers the question. Incubation runs to symptoms, latent to infectiousness.
- C. Serial interval; generation time — reverses the two definitions.
- D. Latent period; communicable period — again names real terms in the wrong places; the communicable period is how long a case remains infectious.
Remember
Generation from infection, serial from symptoms.
Go deeper
A serial interval shorter than the incubation period means transmission occurs before symptoms - which is what makes a disease hard to contain by isolation.
The basic reproduction number and the generation time together determine how fast an epidemic grows, not the reproduction number alone.
In measles the latent period is shorter than the incubation period, which is why a child is infectious for several days before the rash appears.
Park K. Park's Textbook of Preventive and Social Medicine, 28th ed. (Bhanot, 2025).