A hypothetical timeline
Person A is infected on day 0 and develops symptoms on day 4. A infects B on day 3; B develops symptoms on day 8.
| Quantity | Calculation |
|---|---|
| A's incubation period | 4 days |
| B's incubation period | 5 days |
| Generation interval | Day 3 minus day 0 = 3 days |
| Serial interval | Day 8 minus day 4 = 4 days |
The serial and generation intervals differ because the two incubation periods differ.
Observation is easier for some dates
Symptom onset can often be reported, though recall is imperfect. Exact infection time is usually less directly observed. That is one reason serial intervals may be used to inform models, with assumptions that need care.
A negative serial interval is possible
If the person infected second develops symptoms before the person who infected them, the symptom-onset difference can be negative. That does not mean infection traveled backward in time; the incubation periods differ.
Use disease-specific evidence
These intervals vary across pathogens and circumstances and are distributions rather than one guaranteed number. Outbreak interpretation and transmission models need appropriate estimates. A single average should not be treated as a precise deadline for every individual exposure or symptom history.
