While passive surveillance relies on spontaneous or voluntary reporting, active surveillance involves proactive, organized, and systematic efforts to detect, collect, and assess information about adverse drug reactions (ADRs) or vaccine-related events. It is considered more reliable for assessing causality, estimating incidence rates, and evaluating drug safety in specific populations.
Active Surveillance
Active surveillance is critical for understanding the real-world safety profile of drugs and vaccines, especially those newly introduced to the market, used in vulnerable populations, or associated with potential safety signals. The key advantage of active surveillance is its ability to generate more accurate and comprehensive data through targeted monitoring and follow-up.
1. Sentinel Sites
Sentinel surveillance involves monitoring a selected network of healthcare institutions (e.g., hospitals, clinics, or primary care centers) known as sentinel sites. These sites are chosen based on their capability, representativeness, and infrastructure to provide high-quality clinical and pharmacovigilance data.
This approach allows for intensive monitoring of medicine or vaccine use, adverse events, treatment outcomes, and disease patterns within a defined population.
How Sentinel Surveillance Works
- Sites report all adverse events related to a specific medicine or vaccine.
- Healthcare professionals are trained to actively detect and record events.
- Data collection is often standardized, systematic, and periodically analyzed.
- Some sentinel programs are linked with electronic health record (EHR) systems.
Use Cases
- Monitoring of new vaccine introductions (e.g., HPV, rotavirus, COVID-19 vaccines).
- Assessing adverse pregnancy outcomes associated with medicine use.
- Evaluating long-term safety signals such as autoimmune reactions or neurological complications.
Examples
- The US FDA’s Sentinel Initiative: A national system using EHR and insurance claims data from partnered organizations to assess post-marketing drug safety.
- Vaccine Safety Datalink (VSD) in the USA: A collaboration between the CDC and healthcare organizations that monitors immunization safety.
- WHO Sentinel Surveillance for Adverse Events Following Immunization (AEFI): Implemented in low- and middle-income countries to support vaccine safety.
Advantages
- Provides more robust epidemiological data compared to passive reporting.
- Enables active detection and timely analysis of safety signals.
- Supports longitudinal monitoring over time in real-world settings.
Limitations
- May not be nationally representative due to limited number of sites.
- Requires substantial resources, training, and infrastructure.
- Potential for selection bias if sentinel populations are not diverse.
2. Drug Event Monitoring (DEM)
Drug Event Monitoring (DEM) is an active post-marketing surveillance approach that systematically monitors adverse drug reactions in a defined population of patients receiving a specific medication. The method involves prospective follow-up of patients, often through prescription-event linkage systems, questionnaires, or telephone interviews.
DEM is particularly valuable in assessing new drugs or formulations, high-risk medications, or those with limited pre-marketing safety data.
Key Components of DEM
- Identification of patients through prescription databases or hospital records.
- Structured collection of clinical and adverse event data over time.
- Data analysis to evaluate the frequency, severity, and outcomes of ADRs.
- Feedback to prescribers and regulatory bodies.
Use in Vaccine Safety
While DEM is more commonly used for medicinal drugs, it can also be adapted to vaccines, especially when administered to vulnerable groups such as pregnant women, neonates, or the elderly.
Example Programs
- Prescription Event Monitoring (PEM) by the Drug Safety Research Unit (DSRU), UK.
- Intensive Monitoring Programs for antiretroviral drugs and tuberculosis treatments in low- and middle-income countries.
- In India, pilot programs under PvPI have explored prescription-event linkage for select antibiotics and oncology products.
Benefits of DEM
- Enables collection of real-world evidence (RWE) from routine clinical practice.
- Provides comprehensive safety profiles including rare and delayed reactions.
- Can capture data on drug-drug interactions, co-morbidities, and polypharmacy.
Drawbacks
- Cost-intensive: Requires trained staff, databases, and communication mechanisms.
- May suffer from loss to follow-up or recall bias in patient interviews.
- Data collection may not be as timely as automated systems.
3. Registries
A registry is an organized system that uses observational methods to collect uniform data (clinical and demographic) on patients who share a common disease, exposure (e.g., a specific drug or vaccine), or condition over a defined period. Registries are often prospective and longitudinal, enabling long-term safety monitoring and outcome tracking.
In pharmacovigilance, registries are used to assess the real-world use, safety, and effectiveness of products, especially in populations not adequately represented in clinical trials.
Types of Registries:
- Disease Registries – Focused on patients with a specific diagnosis (e.g., rheumatoid arthritis, cancer).
- Exposure Registries – Focused on patients exposed to a particular medicine or vaccine.
- Pregnancy Registries – Track outcomes in women exposed to medicines or vaccines during pregnancy.
Functions in Vaccine and Drug Safety:
- Evaluate long-term outcomes (e.g., autoimmune disorders, malignancies).
- Monitor safety in special populations (children, elderly, pregnant women).
- Provide data for risk-benefit assessment.
- Support labeling updates or regulatory decisions.
Examples
- Antiretroviral Pregnancy Registry (APR): Tracks fetal outcomes after prenatal exposure to antiretroviral medications.
- COVID-19 Vaccine Safety Registries: Set up in multiple countries to monitor outcomes such as thrombosis, myocarditis, or menstrual irregularities.
- Cancer Drug Registries: Monitor new oncology treatments’ safety and effectiveness in the general population.
Advantages
- Enables data collection over extended periods, revealing delayed adverse effects.
- Useful for subgroup analyses (e.g., ethnic differences, comorbidities).
- Enhances external validity and generalizability of safety findings.
Challenges
- Require long-term funding, management, and patient compliance.
- Potential data quality issues if entry is inconsistent or incomplete.
- May face regulatory or ethical concerns, especially in multicenter or global registries.
Comparative Overview: Passive vs. Active Surveillance
| Feature | Passive Surveillance | Active Surveillance |
| Data Collection | Voluntary, spontaneous | Systematic, structured |
| Event Detection | Relies on individual reporting | Actively sought by researchers |
| Cost | Low to moderate | High (staff, infrastructure required) |
| Data Quality | Variable, may be incomplete | More complete and clinically validated |
| Incidence Rate Estimation | Not feasible | Possible due to known denominator |
| Signal Detection | Good for rare/unexpected events | Better for common, delayed, or cumulative AEs |
| Suitability for Special Pop. | Limited | Excellent (e.g., registries, pregnancy studies) |
| Timeliness | Delayed | More timely depending on setup |
Conclusion
Active surveillance is an indispensable complement to passive pharmacovigilance methods. Approaches like sentinel site monitoring, drug event monitoring, and registries provide structured, reliable, and population-specific data that help regulators and researchers understand the full spectrum of a product’s safety profile. Although resource-intensive, these strategies enable more accurate incidence estimation, risk characterization, and long-term safety evaluation, making them essential components of modern pharmacovigilance systems.
By integrating both passive and active systems, national and global health agencies can more effectively protect public health, ensure safe therapeutic practices, and maintain confidence in medical products, especially vaccines.