What is the period of communicability in infective epidemiology
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Period of Communicability
Definition
The communicable period (synonymous with "period of communicability") is defined as:
"The time during which an infectious agent may be transferred directly or indirectly from an infected person to another person, from an infected animal to man, or from an infected person to an animal, including arthropods."
- Park's Textbook of Preventive and Social Medicine
Key Characteristics
1. Variability across diseases
Communicability differs markedly between diseases. Some diseases are MORE communicable during the incubation period (before symptoms appear) than during actual clinical illness. This is why pre-symptomatic transmission is epidemiologically significant.
2. Relationship to incubation period
The period of communicability does not always coincide neatly with the incubation period:
- In measles: communicable from about 4 days before to 4 days after the appearance of the rash (i.e., spans the prodrome and early illness).
- In mumps: maximum communicability reaches its height ~48 hours before onset of salivary gland swelling.
- In malaria: communicable as long as viable gametocytes exist in circulating blood (vector-dependent communicability).
- In tuberculosis: communicable over a prolonged period (weeks to years), so duration of exposure matters more than a single event.
- In Ascaris (roundworm): communicable until all fertile females are destroyed by treatment.
3. Effect of treatment
Early diagnosis and treatment can shorten or eliminate the communicable period for many infections (e.g., antibiotics for streptococcal pharyngitis reduce communicability within 24 hours).
Related Concepts
| Concept | Definition |
|---|---|
| Incubation period | Time from receipt of infection to appearance of first symptoms |
| Serial interval | Gap in time between onset in the primary case and onset in secondary cases |
| Generation time | Interval between receipt of infection by a host and maximal infectivity of that host |
| Secondary attack rate (SAR) | The number of exposed susceptible persons developing disease within one incubation period after exposure to a primary case |
Generation time vs. communicable period: Generation time is defined as the interval from receipt of infection to maximal infectivity - it marks the peak of communicability, whereas the communicable period spans the full window during which transmission is possible.
Secondary Attack Rate (SAR) - The Measure of Communicability
SAR quantifies how efficiently a disease spreads during the communicable period:
$$\text{SAR} = \frac{\text{Number of exposed susceptibles developing disease within 1 incubation period}}{\text{Total number of exposed susceptible contacts}} \times 100$$
- For short communicable periods (measles, chickenpox): SAR is calculated per single exposure event.
- For prolonged communicable periods (tuberculosis): SAR is expressed per person-weeks or person-months of exposure, since duration of contact matters.
Disease-Specific Examples from the Literature
| Disease | Period of Communicability |
|---|---|
| Measles | 4 days before to 4 days after rash onset |
| Rubella | ~7 days before to ~5-7 days after rash onset |
| COVID-19 | 1-3 days before symptoms; up to 1-2 weeks (mild), 3+ weeks (severe) |
| Malaria | While viable gametocytes in blood (may last 40+ years in P. malariae) |
| Amoebiasis | As long as cysts are excreted (may be years if untreated) |
| Hepatitis E | Unknown |
| Tetanus | None (not person-to-person transmissible) |
Practical Importance in Epidemiology
- Isolation policy - Patients are isolated during the communicable period to prevent transmission.
- Contact tracing - Contacts are traced back to the start of the communicable period, not just symptom onset.
- Outbreak control - Identifying pre-symptomatic communicability helps explain and control explosive outbreaks.
- Quarantine duration - Quarantine of contacts is timed to cover the maximum incubation period of the disease.
- Epidemic curve interpretation - Whether an epidemic is point-source or propagated is determined partly by the communicable period and serial interval.
- Park's Textbook of Preventive and Social Medicine, pp. 112-113
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