Public Health Surveillance

By Dr. Jay K. Varma
Physician, Epidemiologist, and Public Health Expert

👁️ What Is Public Health Surveillance?

Public health surveillance is the ongoing, systematic collection, analysis, and interpretation of health data—used to plan, implement, and evaluate public health programs. In simple terms, it’s how we track the health of a population over time.

Surveillance helps public health professionals answer three essential questions:

• Who is getting sick?

• Where is it happening?

• When did it start?

This data allows health departments to detect outbreaks early, understand trends, and take action to prevent disease from spreading further.

🏛️ Who Is Responsible for Surveillance?

In the United States, surveillance happens at multiple levels of government:

• National – The Centers for Disease Control and Prevention (CDC)

• State – Each state has a health department (e.g., New York State Department of Health)

• Local – Cities like New York have their own departments (e.g., NYC Department of Health and Mental Hygiene)

Each jurisdiction has legal responsibility for monitoring certain diseases. This includes collecting, analyzing, and reporting data to ensure timely responses to threats.

🦠 Surveillance of Cases

Each person who meets criteria for a particular disease is called a case.

In New York, for example, there are about 80 diseases on a list that must be reported by law. Here’s how reporting works:

• Clinicians must report diagnoses via phone, fax, or online systems.

• Laboratories are required to submit results electronically.

For infectious diseases, laboratory reports are considered the gold standard, because they provide high confidence that a person truly has the disease. These labs are strictly regulated for accuracy. 

⚰️ Surveillance of Deaths

Every death must be documented through a death certificate, which lists both the fact of death and its cause. Physicians complete these certificates, and in some places like New York City, the system is now entirely electronic.

However, in many parts of the U.S.—and in much of the world—death data can be incomplete or inaccurate. This creates gaps in understanding what people are dying from and why.

Still, death data remains a critical signal in disease surveillance, especially when monitoring epidemics like COVID-19 or influenza.

🧾 Syndromic Surveillance

Not all surveillance relies on lab-confirmed diagnoses. Sometimes, public health needs faster signals.

Emergency departments submit reports about symptoms, not confirmed illnesses. This is known as syndromic surveillance. For instance:

• “Influenza-like illness” may be defined as fever plus sore throat or fever plus cough

• A rise in diarrheal illness could point to a foodborne outbreak, even without confirmed lab results

Syndromic systems are more sensitive but less specific—they can detect signals early, but include more false positives.

🩺 The Role of the Astute Clinician

For rare but dangerous conditions like anthrax, botulism, or Ebola, the most reliable system is often just a phone call.

An “astute clinician” who sees something unusual must know how to contact their local public health authority. These early alerts are how many major threats are first identified.

You never want a public health emergency to be the first time you meet your local health department.

⚖️ Balancing Sensitivity and Specificity

Each method of surveillance has trade-offs:

Surveillance Method Strength Weakness

Lab-confirmed Very specific Misses untested cases

Syndromic Sensitive and fast Less specific

Death reporting Broad significance Delayed and sometimes inaccurate

Public health officials rely on a composite picture from all systems to understand what’s happening and respond effectively.

🔗 Learn More

📖 Improving infectious disease surveillance in China

📖 Africa Centres for Disease Control and Prevention Is Closing Gaps in Disease Detection

📖Zika virus surveillance in New York City