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# Public health surveillance

- Concept

Surveillance collects, analyses and shares health data continuously to spot problems and act in time.

Updated Nov 5, 2026

## Key points

- Passive: cheap and wide, but incomplete and slow; the provider takes the initiative.
- Active: more complete but costly; used for a limited time, in outbreaks or for high-priority events.
- Sentinel: a few selected sites that report consistently; useful for trends, though it may not represent the whole population.
- Syndromic: uses symptoms, emergency visits or medicine sales to detect signals before diagnosis.
- A good system is timely, representative, sensitive and specific (and also simple and acceptable).
- In Colombia, SIVIGILA, run by the National Institute of Health, receives reports of events of public health interest.

## What it is

Public health surveillance is the systematic, ongoing collection of health data, their analysis and interpretation, and their dissemination to those who must act. It can be passive (providers report under the rules), active (the health authority goes looking for cases), sentinel (a fixed set of sites reports all cases) or syndromic (it uses pre-diagnostic data, such as visits for fever). Every system has some under-reporting.

## Worked example

- Data (made up): in June, passive reporting in a municipality records 40 leptospirosis cases.
- Active search: the team reviews clinical records and laboratories and finds 100 cases that meet the definition.
- Completeness of passive reporting: 40 / 100 = 40 %.
- Under-reporting factor: 100 / 40 = 2.5; for each reported case there were 2.5 real cases.
- Syndromic surveillance: emergency visits for fever rise from 120 to 210 a week.
- Increase: (210 − 120) / 120 = 0.75, that is, 75 % more visits, a signal worth investigating.
- Reading: passive surveillance saw fewer than half the cases; the syndromic signal raised the alarm before confirmed diagnoses.

## Types of surveillance

| Type | Who starts it | Strength | Limitation |
| --- | --- | --- | --- |
| Passive | The provider reports | Cheap, covers the whole territory | Under-reporting, delay |
| Active | The authority looks for cases | More complete | Costly, temporary |
| Sentinel | Selected sites | Consistent, good-quality data | May not represent the population |
| Syndromic | Automated systems with pre-diagnostic data | Early detection | Many false alarms, not specific |

## Markers

- Passive
- Active
- Sentinel
- Syndromic
- SIVIGILA

## In practice

- Reporting notifiable events is a doctor's duty in Colombia: without it the system cannot see the outbreak.
- Surveillance data serve to estimate disease burden, detect outbreaks, evaluate programmes (such as vaccination) and direct resources.

## Common pitfalls

- Reading a rise in reported cases as a real rise without checking for changes in the case definition or in access to testing.
- Forgetting that under-reporting is not the same everywhere: places with fewer services seem to have less disease.

## Memory aid

Wait, Search, Watch from a tower, Smell the smoke: passive waits, active searches, sentinel watches from fixed posts, syndromic smells smoke before the fire.

## Test yourself

Think of the answer, then tap the question to see it.

## How does passive surveillance differ from active surveillance?

In passive surveillance providers report on their own under the rules; in active surveillance the health authority contacts providers and looks for cases.

## What is sentinel surveillance?

A fixed group of sites (clinics, hospitals) that agrees to report every case of an event; it gives consistent data, though not always representative.

## What kind of data does syndromic surveillance use?

Pre-diagnostic data: symptoms, emergency visits, school absences or medicine sales.

## Why is a rise in reported cases not always a real rise in disease?

Because the case definition may have changed, testing may have increased or reporting may have improved.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/epidemiology/public-health-surveillance/)

## Connections

## Related 5

- [SIVIGILA: public health surveillance in Colombia](https://durgical.com/en/epidemiology/sivigila-public-health-surveillance-in-colombia/)
- [Notifiable events in Colombia](https://durgical.com/en/epidemiology/notifiable-events-in-colombia/)
- [Outbreak investigation](https://durgical.com/en/epidemiology/outbreak-investigation/)
- [Endemic, epidemic, outbreak and pandemic](https://durgical.com/en/epidemiology/endemic-epidemic-outbreak-and-pandemic/)
- [Epidemic curve](https://durgical.com/en/epidemiology/epidemic-curve/)

## Related conditions 1

- [Dengue](https://durgical.com/en/conditions/dengue/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology: Lesson 5, Section 2](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson5/section2.html) (Centers for Disease Control and Prevention, 2012); [Principles of Epidemiology: Lesson 5, Section 4](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson5/section4.html) (Centers for Disease Control and Prevention, 2012); [SIVIGILA](https://www.ins.gov.co/Direcciones/Vigilancia/Paginas/SIVIGILA.aspx) (Instituto Nacional de Salud, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/epidemiology/public-health-surveillance/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/epidemiology/public-health-surveillance/