Disease Surveillance & Notification

Surveillance is the eyes and ears of public health — the continuous, systematic collection, analysis and interpretation of health data, tied to timely dissemination so that action can be taken. This topic is a perennial favourite in NEET PG/INI-CET because it rewards rote recall of definitions, India-specific programmes (IDSP/IHIP), notifiable disease lists, reporting forms (P, S, L) and crisp WHO terminology.

Definition & core concept

Surveillance, as defined by the CDC/WHO, is the ongoing systematic collection, analysis, interpretation, and dissemination of health data for planning, implementation and evaluation of public health practice, closely integrated with timely dissemination to those who need to know so that action can be taken.

The operational mantra is the "information for action" loop:

Data collection → Compilation/consolidation → Analysis & interpretation → Dissemination/feedback → Public-health action

A surveillance system that collects data but does not generate feedback or action is, by definition, a failed system. Alexander Langmuir is regarded as the "father of modern surveillance".

High-yield: Surveillance = "information for action". The single most-tested distinguishing feature is that the loop must close with feedback and action — mere data collection (as in a survey or census) is not surveillance because surveillance is continuous whereas a survey is a one-time snapshot.

Surveillance vs survey vs screening

Feature	Surveillance	Survey	Screening
Timing	Continuous, ongoing	Cross-sectional, one-time	Presumptive identification
Population	Whole population/defined group	Sample	Apparently healthy persons
Purpose	Trend detection, action	Estimate prevalence/burden	Detect disease early
Output	Feedback loop & action	Report/estimate	Refer test-positive for diagnosis

Objectives of surveillance

1. Detect epidemics/outbreaks early (alert and response).
2. Estimate magnitude of a health problem (incidence, prevalence, mortality).
3. Determine geographic & temporal distribution (place & time trends).
4. Identify high-risk groups and risk factors.
5. Evaluate control/prevention programmes and predict epidemics.
6. Generate hypotheses and stimulate research.

Classification of surveillance

Surveillance is classified along several axes that examiners love to cross-question.

1. Active vs passive surveillance

Feature	Active surveillance	Passive surveillance
Direction of data flow	Health agency goes and seeks the data	Health facilities report routinely to agency
Effort/cost	Labour-intensive, expensive	Cheap, easy to maintain
Completeness	High, accurate, few missed cases	Under-reporting common
Example	House-to-house search in polio/smallpox eradication, outbreak investigation, Pulse Polio	Routine IDSP reporting, notifiable disease reporting
Use	Eradication programmes, outbreak settings	Long-term routine monitoring

High-yield: Active surveillance is the type used in eradication programmes (smallpox, polio) and during outbreak investigations because it minimises missed cases. Passive surveillance is the routine, cheap, default mode but suffers under-reporting. The classic exam answer for "house-to-house case search" = active surveillance.

2. Sentinel surveillance

Data are collected from selected reporting sites (sentinel sites) chosen for their probability of detecting the condition — not from the entire population. It is cost-effective and used when comprehensive surveillance is impractical.

• Typical examples: influenza sentinel sites, HIV sentinel surveillance (antenatal clinics & STI clinics used as sentinel groups), injuries, rotavirus.
• Limitation: may miss rural/peripheral cases and is not representative of the whole population; cannot give true incidence for the entire community.

High-yield: HIV Sentinel Surveillance in India uses ANC (antenatal clinic) attendees as a proxy for the general population and STI clinic / high-risk group sites for concentrated epidemics. A sentinel site is chosen for quality and probability of detection, not for representativeness.

3. Other patterns

• Sero-surveillance — antibody/antigen monitoring in serum samples (e.g., serosurveys for COVID-19, measles immunity).
• Syndromic surveillance — based on clinical syndromes (e.g., "acute febrile illness", ILI/SARI) before lab confirmation; allows early outbreak detection.
• Surveillance of vital events — births and deaths.
• Demographic & event-based surveillance — rumour/media-based early warning.

Integrated Disease Surveillance Programme (IDSP) — India

IDSP is the backbone of decentralised, laboratory-based, IT-enabled disease surveillance in India. It is the single most exam-relevant Indian programme on this topic.

• Launched: 2004 (with World Bank assistance), under the National Health Mission, NCDC (National Centre for Disease Control), Delhi.
• Aim: Strengthen/maintain a decentralised, lab-based, IT-enabled disease surveillance system to monitor disease trends and detect & respond to outbreaks early.
• Surveillance Units established at Central (CSU), State (SSU) and District (DSU) levels.
• The Central Surveillance Unit (CSU) is at NCDC, integrating data from State and District units.

The reporting forms — P, S, L

This is one of the most directly asked single-best-answer questions.

Form	Filled by	Type of data
P form (Presumptive)	Health worker / paramedical / sub-centre staff	Syndromic / presumptive (based on symptoms)
S form (Suspect)	Medical officer / clinician (PHC, hospital)	Clinical / probable diagnosis
L form (Laboratory)	Laboratory staff	Laboratory-confirmed data

High-yield: P = Presumptive (health worker, syndromic), S = Suspected (clinician/medical officer, clinical), L = Laboratory (lab-confirmed). Mnemonic: "PSL = Para-worker, Surgeon/clinician, Lab." This is the classic IDSP MCQ.

Surveillance conditions under IDSP

IDSP tracks regular surveillance (vector-borne — malaria, dengue, chikungunya; water-borne — acute diarrhoeal disease, typhoid, cholera, viral hepatitis; respiratory — TB; vaccine-preventable — measles; others) and a list of non-communicable disease risk factors (added later — NCD risk factor surveillance, e.g., diabetes, hypertension, tobacco/alcohol use).

IHIP — the modern upgrade

High-yield: IDSP is being migrated to the IHIP — Integrated Health Information Platform, a near-real-time, case-based, individual-level, web-enabled surveillance system (one of the world's largest online disease surveillance platforms). IHIP captures GIS-tagged, individual data versus IDSP's aggregate weekly data and covers 33 diseases (expanded). Examiners now ask "real-time case-based digital surveillance in India = IHIP."

Outbreak response

When the P/S/L data cross a threshold, the Rapid Response Team (RRT) at district level is activated for outbreak investigation and containment. Weekly data are submitted on a fixed reporting day; a media-scanning/event-based component supplements the indicator-based system.

Notification of diseases

Notification is the legal/statutory process of informing designated public-health authorities about the occurrence of a specified ("notifiable") disease. It enables containment, contact tracing and resource mobilisation.

Internationally notifiable — IHR (2005)

Under the International Health Regulations (IHR 2005), three diseases are notifiable to WHO unconditionally/always (a single confirmed case must be reported):

1. Smallpox
2. Poliomyelitis (wild-type poliovirus)
3. Human influenza caused by a new subtype (e.g., novel influenza)
4. SARS (Severe Acute Respiratory Syndrome)

Other events (cholera, plague, yellow fever, viral haemorrhagic fevers, West Nile, etc.) are notified using the IHR decision instrument/algorithm (any event that may constitute a Public Health Emergency of International Concern — PHEIC).

High-yield: Under IHR 2005, the diseases notifiable regardless of context (always) are smallpox, wild-type poliomyelitis, human influenza due to a new subtype, and SARS. The remainder are assessed using the decision-instrument algorithm to decide if they constitute a PHEIC, declared by the WHO Director-General.

Historic note: Under the old International Sanitary/Health Regulations the internationally quarantinable diseases were cholera, plague, yellow fever (and formerly smallpox, now eradicated). This older list is still sometimes asked.

Notifiable diseases in India

There is no single uniform national notifiable-disease list — notification is largely a State subject, so the list varies by State/UT. However, several diseases/programmes mandate notification nationally.

Diseases commonly mandated for notification in India include:

• Tuberculosis — made a notifiable disease in 2012 (notification to Nikshay portal; private practitioners are legally bound to notify; failure can attract penal provisions). HIGH-YIELD.
• HIV/AIDS notification provisions, and notification under specific programmes.
• Plague, cholera, yellow fever (quarantinable).
• Diseases under the Epidemic Diseases Act, 1897 when an epidemic is declared.
• COVID-19 was notified under the Epidemic Diseases Act, 1897 + Disaster Management Act, 2005.

High-yield: Tuberculosis became a notifiable disease in India in 2012; notification is to the Nikshay portal and is mandatory for private practitioners and pharmacists/labs. This is the single most repeated India-notification MCQ.

High-yield: The legal teeth for epidemic control in India is the Epidemic Diseases Act, 1897 (used for plague historically and for COVID-19 recently). Public-health notification is otherwise a State subject, hence lists differ between States.

Levels of surveillance & how they interact (India)

The three-tier flow in India's public-health surveillance:

Sub-centre / Peripheral (P form, health worker) → Primary Health Centre (S form, medical officer) → District Surveillance Unit (DSU) + District lab (L form) → State Surveillance Unit (SSU) → Central Surveillance Unit (CSU, NCDC) → feedback & RRT action

• Peripheral level: case detection, reporting (P/S forms), immediate local action.
• Intermediate (District/State): data consolidation, lab confirmation (L form), outbreak verification, RRT deployment.
• Central level (CSU/NCDC): national analysis, trend monitoring, policy, IHR reporting to WHO via the National IHR Focal Point.

Attributes / evaluation of a surveillance system (CDC)

Frequently asked as "which attribute…?" — know these key ones:

Attribute	Meaning
Sensitivity	Proportion of true cases detected by the system
Specificity / Predictive value positive (PVP)	Proportion of reported cases that are truly cases
Timeliness	Speed between steps (detection → report → response)
Representativeness	Accurately describes occurrence over time, place, person
Simplicity	Structure & ease of operation
Flexibility	Adapts to new conditions/needs
Acceptability	Willingness of participants to report
Stability	Reliability & availability
Data quality	Completeness & validity of recorded data

High-yield: A system that catches most true cases has high sensitivity; a system in which most reported cases are genuinely cases has high predictive value positive (PVP). Active surveillance maximises sensitivity; passive surveillance often has low sensitivity (under-reporting) but may still have adequate PVP.

Special surveillance terms (rapid recall)

• AFP surveillance — Acute Flaccid Paralysis surveillance is the gold standard for polio surveillance; target detection rate ≥ 1 (now ≥2 in endemic-risk settings) per 100,000 children < 15 years, with adequate stool samples (2 samples ≥ 24 h apart within 14 days of onset). Non-polio AFP rate is a sensitivity indicator.
• VPD surveillance — vaccine-preventable disease (measles–rubella case-based surveillance).
• AEFI surveillance — Adverse Events Following Immunization.
• Behavioural surveillance survey (BSS) — tracks risk behaviours (HIV).
• One Health surveillance — integrating human, animal and environmental data.

Recently asked / exam angle

• P, S and L forms of IDSP — match the form to who fills it (health worker = P/presumptive; medical officer = S/suspected; lab = L). Easiest 1-marker.
• Active vs passive — "house-to-house case search in eradication = active surveillance"; "routine reporting from PHC = passive."
• TB notification year (2012) and Nikshay — recurring.
• IHR 2005 always-notifiable four (smallpox, polio, novel influenza, SARS) and PHEIC declared by WHO DG.
• IDSP launched 2004; CSU at NCDC; migrating to IHIP (real-time, case-based) — IHIP is the new buzzword.
• Sentinel surveillance using ANC clinics for HIV — definition-based.
• Father of surveillance = Langmuir; "information for action".
• AFP surveillance = polio; non-polio AFP rate as a sensitivity indicator.
• Difference between surveillance and survey (continuous vs one-time).
• CDC attributes — sensitivity, timeliness, representativeness, PVP.

Key differentials / commonly confused pairs

• Notification vs Surveillance: Notification is a legal reporting act for specified diseases; surveillance is the broader ongoing data-action system (notification feeds passive surveillance).
• Sentinel vs Sample survey: Sentinel = ongoing data from selected high-yield sites; survey = one-time sample of the population.
• Active vs Sentinel: Active = actively seek all cases (eradication); sentinel = passive-style reporting but only from chosen sites.
• IDSP vs IHIP: IDSP = aggregate, weekly, paper/portal; IHIP = individual, near-real-time, case-based, GIS-tagged web platform.

Complications / limitations of surveillance

• Under-reporting (especially passive) → underestimation of burden.
• Reporting delay → late outbreak response.
• Lack of feedback loop → demotivated reporters, system collapse.
• Selection bias in sentinel sites (not representative).
• Inconsistent State notifiable lists in India → fragmented data.
• Resource and IT-connectivity constraints at peripheral levels.

Rapid revision

1. Surveillance = continuous data collection + analysis + feedback/action ("information for action"); Langmuir is its father.
2. Active surveillance = agency seeks data (eradication/outbreak, e.g., polio house-to-house); passive = routine reporting (cheap, under-reports).
3. Sentinel surveillance uses selected high-yield sites; HIV uses ANC clinics; not population-representative.
4. Syndromic surveillance = based on clinical syndromes before lab confirmation → earliest outbreak detection.
5. IDSP launched 2004, under NCDC; units at Central (CSU), State (SSU), District (DSU) levels.
6. P = Presumptive (health worker), S = Suspected (clinician), L = Laboratory forms — mnemonic PSL.
7. IDSP → IHIP (Integrated Health Information Platform): near-real-time, case-based, individual-level, web/GIS surveillance.
8. IHR 2005 always-notifiable four: smallpox, wild poliovirus, human influenza (new subtype), SARS; others judged by the decision algorithm for PHEIC (declared by WHO DG).
9. TB notifiable in India since 2012 → report to Nikshay; mandatory for private practitioners.
10. Epidemic Diseases Act, 1897 is the legal backbone (plague, COVID-19); notification is otherwise a State subject.
11. AFP surveillance = gold standard for polio; non-polio AFP rate is a sensitivity indicator.
12. CDC attributes to evaluate a system: sensitivity, PVP, timeliness, representativeness, simplicity, flexibility, acceptability, stability, data quality.