Public health surveillance is the ongoing, systematic collection, analysis, interpretation, and dissemination of health data for planning, implementing, and evaluating public health action. Its motto is "information for action": data that are collected but not used for response are not surveillance.
Types of surveillance
| Type | Description | Example |
|---|
| Passive | Health facilities report cases they see; the health office waits for reports | Routine weekly reports from RHUs and hospitals |
| Active | Health workers go out to search for cases | House-to-house search for measles or acute flaccid paralysis cases during an outbreak |
| Sentinel | Selected facilities report detailed data on selected conditions | Designated sentinel hospitals reporting a selected syndrome such as influenza-like illness |
| Syndromic | Reporting of symptom groups before diagnosis is confirmed | Acute watery diarrhea, influenza-like illness |
| Indicator-based | Structured reporting of defined diseases using case definitions | PIDSR |
| Event-based | Rapid capture of unstructured reports of unusual events (rumors, media, community reports) | Event-based surveillance and response (ESR) reports of clusters or unusual deaths |
Key definitions
- Notifiable disease — a disease that, by legal requirement, must be reported to public health authorities
- Outbreak / epidemic — occurrence of cases clearly in excess of what is normally expected in a given area or group over a given period
- Cluster — cases grouped in place and time, suspected to be greater than expected
- Case definition — standard criteria (clinical, laboratory, person, place, time) for deciding who counts as a case; usually graded as suspect, probable, and confirmed
- Alert threshold — level of cases that triggers early warning; epidemic (outbreak) threshold — level above the expected range that triggers response
Philippine legal and system framework
- RA 11332 — Mandatory Reporting of Notifiable Diseases and Health Events of Public Health Concern Act (approved April 26, 2019). It repealed Act No. 3573, the old Law on Reporting of Communicable Diseases.
- Who must report: all public and private physicians, allied medical personnel, hospitals, clinics, health facilities, laboratories, workplaces, schools, prisons, ports, airports, establishments, communities, other government agencies, and NGOs must report notifiable diseases and health events of public health concern, as issued by the DOH, accurately and immediately.
- Epidemiology and Surveillance Units (ESUs) must be established and functional at all levels of the DOH and its local counterparts, in public and private health facilities and laboratories, and in ports and airports.
- Declaration: the Secretary of Health declares epidemics of national and/or international concern; when the threat endangers national security, the President declares a State of Public Health Emergency.
- Prohibited acts include unauthorized disclosure of private and confidential patient information, tampering with records or intentionally providing misinformation, non-operation of the required surveillance and response system, and non-cooperation of persons or entities that must report or are identified as having a notifiable disease.
- Penalty: a fine of ₱20,000 to ₱50,000, or imprisonment of 1 to 6 months, or both, at the court's discretion; professional licenses and civil service eligibility may also be suspended or revoked.
- The Act recognizes the FHSIS and the PIDSR (with case-based and event-based surveillance) among the official public health information systems.
- PIDSR — Philippine Integrated Disease Surveillance and Response (framework issued through DOH Administrative Order 2007-0036). It monitors diseases and syndromes with outbreak potential that need immediate response.
- Category I — immediately notifiable: report within 24 hours of detection, with disease-specific case investigation forms, from the city/municipality or hospital up to the provincial and regional levels. Examples: acute flaccid paralysis, measles-rubella, neonatal tetanus, rabies, meningococcal disease, adverse events following immunization; any outbreak, cluster, or unusual disease or threat is also reported immediately.
- Category II — weekly notifiable: reported weekly on case report forms to the next higher level, following the current DOH schedule. Examples: dengue, leptospirosis, typhoid and paratyphoid fever, cholera, acute bloody diarrhea, influenza-like illness.
- The DOH updates the list of notifiable conditions; always follow the current DOH list.
- ESUs operate at regional (RESU), provincial (PESU), and city (CESU) levels, with hospitals and other facilities serving as disease reporting units.
- International Health Regulations (IHR 2005): the country notifies the WHO of events that may constitute a public health emergency of international concern.
Steps of an outbreak investigation (steps may overlap; control measures start as soon as a likely source is known)
- Prepare for field work (team, supplies, forms, coordination with the ESU and local officials)
- Establish the existence of an outbreak — compare current cases with expected numbers; rule out artifacts (new reporting, new test)
- Verify the diagnosis — clinical review and laboratory confirmation
- Construct a working case definition
- Find cases systematically and record them in a line list
- Describe the outbreak by time, place, and person (epidemic curve, spot map, attack rates by age and sex)
- Develop hypotheses
- Evaluate hypotheses — analytic study (case-control or retrospective cohort)
- Refine hypotheses and do additional studies (laboratory, environmental)
- Implement control and prevention measures
- Maintain surveillance to confirm the outbreak is over
- Communicate findings (report to authorities, feedback to the community)
Epidemic curve patterns
| Pattern | Shape | Meaning |
|---|
| Point source | One sharp peak; cases within one incubation period | Single exposure (e.g., one contaminated meal) |
| Continuous common source | Rapid rise then plateau | Ongoing exposure (e.g., contaminated water supply) |
| Propagated (person-to-person) | Successive, progressively taller peaks about one incubation period apart | Person-to-person spread (e.g., measles) |
Principles of communicable disease control: eliminate or treat the source/reservoir, interrupt transmission, and protect susceptible hosts (immunization, prophylaxis).
Attack rate (AR) = number of new cases during the outbreak ÷ population at risk at the start × 100. It is an incidence proportion used for short outbreak periods.
Worked example — fiesta food poisoning. 120 guests ate at a barangay fiesta; 52 became ill within 12 hours.
- Overall AR = 52 ÷ 120 × 100 = 43.3% (check: 52 ÷ 120 = 0.4333 ✓)
| Food | Ate: ill / total | AR ate | Did not eat: ill / total | AR not ate | RR |
|---|
| Chicken | 48 / 80 | 60% | 4 / 40 | 10% | 6.0 |
| Pancit | 30 / 70 | 42.9% | 22 / 50 | 44% | 0.97 |
- Chicken: 48 ÷ 80 = 0.60; 4 ÷ 40 = 0.10; RR = 0.60 ÷ 0.10 = 6.0 ✓ (48 + 4 = 52 ill ✓; 80 + 40 = 120 ✓)
- Pancit: 30 ÷ 70 = 0.4286; 22 ÷ 50 = 0.44; RR = 0.4286 ÷ 0.44 = 0.97 ✓ (30 + 22 = 52 ✓; 70 + 50 = 120 ✓)
- Interpretation: the chicken is the likely vehicle — high AR among those who ate it, low AR among those who did not, and most cases ate it. Pancit shows no difference.
Secondary attack rate (SAR) = new cases among exposed susceptible contacts within one incubation period ÷ number of susceptible contacts × 100 (primary cases and already-immune contacts are excluded from the denominator).
- Example: five households each have one measles case. The households contain 30 other members, 10 of whom are already immune. Of the 20 susceptible contacts, 5 develop measles within one incubation period.
- SAR = 5 ÷ 20 × 100 = 25% (check: 5 ÷ 20 = 0.25 ✓)
Immediate nursing actions when an outbreak is suspected
- Notify the ESU/municipal health officer at once (Category I: within 24 hours)
- Manage and refer cases; start infection-control measures
- Collect specimens as directed; keep food or water samples
- Begin a line list: name, age, sex, address, date and time of onset, symptoms, exposures, outcome
- Search for additional cases (active case finding)
- Give health education and apply control measures (e.g., boil-water advisory, vaccination of contacts)
- Detector and reporter: recognizes cases using case definitions and reports on time through the proper channel
- Investigator: interviews cases and controls, completes case investigation forms, builds the line list, and plots the epidemic curve and spot map
- Implementer of control measures: contact tracing, immunization, isolation advice, chemoprophylaxis per protocol, environmental measures with sanitary inspectors
- Educator and communicator: gives accurate risk information and counters rumors
- Coordinator: works with BHWs, barangay officials, schools, and the ESU
- Monitor: continues surveillance until cases return to the expected level
- Reporting notifiable diseases is a legal duty, not optional, under RA 11332. Non-reporting, delayed cooperation, or tampering with records is a prohibited act.
- Confidentiality: under RA 11332, surveillance information may be used only for public health purposes by authorized personnel; unauthorized disclosure is punishable. Health information is sensitive personal information under RA 10173.
- Least restrictive measures: quarantine and isolation should be proportionate and respect civil liberties while protecting the public.
- Non-stigmatization: describe exposures and places, not "guilty" individuals or families.
Case 1 — Category I case. A nurse at an RHU sees a 3-year-old with sudden limb weakness (acute flaccid paralysis).
- Correct action: report immediately (within 24 hours) to the ESU, complete the case investigation form, and arrange stool specimen collection as directed.
- Why: AFP is immediately notifiable; one case warrants investigation.
Case 2 — Epidemic curve. Cases of diarrhea appear over 3 weeks, rising quickly and then remaining high each day.
- Interpretation: continuous common-source outbreak, such as a contaminated water source.
- Action: check and treat the water supply and advise boiling or chlorinating drinking water right away.
Case 3 — Confidentiality. A barangay official asks the nurse for the names of households with confirmed cases to post in the barangay hall.
- Correct action: decline; share only aggregate information and prevention advice.
- Why: RA 11332 and RA 10173 protect patient identity.
- Waiting for laboratory confirmation before reporting a Category I suspect case — report suspect cases immediately.
- Delaying control measures until the analytic study is finished.
- Forgetting to verify the outbreak (a spike may come from better reporting).
- Using total household members instead of susceptible contacts for SAR.
- Reading a single sharp peak as person-to-person spread — it indicates a point source.
- Thinking only physicians must report — RA 11332 covers all health workers and many institutions.
- Surveillance = ongoing systematic collection, analysis, interpretation, dissemination → action.
- Passive = wait for reports; active = go find cases; sentinel = selected sites; event-based = unusual events and rumors.
- RA 11332 (2019): mandatory reporting; ESUs at all levels; repealed Act 3573.
- DOH Secretary declares epidemics of national/international concern; President declares a State of Public Health Emergency.
- RA 11332 penalty: ₱20,000–₱50,000 fine or 1–6 months imprisonment, or both.
- PIDSR Category I = within 24 hours (e.g., AFP, measles, neonatal tetanus); Category II = weekly (e.g., dengue, leptospirosis, typhoid).
- Outbreak steps start with verifying the outbreak and diagnosis; control measures begin as early as possible.
- Point source = one peak; continuous common source = plateau; propagated = successive peaks.
- AR = cases ÷ population at risk × 100; SAR uses susceptible contacts only.