Event-based Surveillance: Rapid Detection for EWAR Systems

When a mysterious respiratory illness begins spreading in a community, traditional disease surveillance systems might take weeks to detect and report it. By that time, the outbreak could have already crossed borders. This is where event-based surveillance becomes critical-a rapid detection system that monitors unstructured information sources to identify potential health threats before they escalate into major crises.

What is event-based surveillance? 🔗

Event-based surveillance (EBS) involves the organized approach to detecting and reporting signals that may represent events of public health importance, often through channels outside routine surveillance systems. Unlike traditional indicator-based surveillance (IBS) that relies on structured data from health facilities, EBS actively scans diverse information sources including media reports, community observations, social media posts, and official health notices to detect unusual health patterns.

The power of EBS lies in its ability to detect threats quickly. EBS reporting can occur earlier than IBS and reach populations and geographic regions not adequately covered by traditional surveillance. This early detection capability proved vital during the COVID-19 pandemic when Canada’s Global Public Health Intelligence Network identified the first signals on December 31, 2019-days before official notifications.

How event-based surveillance complements indicator-based surveillance 🔗

EBS and IBS serve different but complementary roles in disease surveillance. IBS primarily relies upon information collected in health facilities and focuses on detecting a list of communicable diseases using standardized case definitions. While excellent for monitoring disease trends over time, IBS has limitations-data can be delayed, incomplete, and may only cover pre-defined diseases.

EBS fills these gaps through several key advantages. It detects both known and unknown diseases, operates independently of healthcare infrastructure, and uses an all-hazards approach that includes chemical, biological, and radiological events. Signals can be designed to detect patterns of disease such as clusters of similar illness in a community or single cases of suspected high-priority events like viral hemorrhagic fever.

The two systems work best together. IBS provides detailed, verified information about disease patterns and transmission, while EBS offers rapid preliminary alerts about emerging threats. During emergencies, EBS can be deployed as an adjunct to national disease surveillance systems, providing early warning while traditional systems catch up.

Signal detection and verification 🔗

EBS systems screen thousands of information pieces daily from multiple sources. Information from both unofficial and official sources is screened using event assessment tools that provide criteria for determining whether information requires further assessment. These criteria include whether the disease can cause outbreaks with high spread potential, involves unusual or unexpected events, or could have consequences for international trade or travel.

The verification process distinguishes credible signals from noise. Information that meets screening criteria undergoes risk assessment using frameworks like the International Health Regulations Annex 2. Events deemed potentially risky receive further review by technical experts across epidemiology, laboratory science, risk communication, and emergency management before action is taken.

Developing an effective EBS strategy 🔗

Implementing EBS requires careful planning around reporting mechanisms and feedback systems. The objective of Early Warning Alert and Response (EWAR) systems is to support early detection and rapid response to acute public health events of any origin.

Establishing reporting mechanisms 🔗

Effective EBS depends on clear reporting pathways from signal detection to response. Multiple information channels must be monitored systematically, including internet-based early warning systems, media sources in various languages, official communications from health authorities, and reports from partner agencies. Event screening is undertaken twice daily, seven days a week, with epidemic intelligence officers, medical officers, and epidemiologists operating the system.

The reporting structure typically involves several layers. Signals detected at the community or facility level flow through local health offices to regional and national authorities. For international events, WHO works with Ministries of Health and health sector partners to train local health workers to use the system, ensuring smooth information flow across borders.

Creating feedback loops for alert management 🔗

Feedback mechanisms ensure that those reporting signals understand the outcomes of their reports. Systems provide regular feedback to reporting facilities including receipt of reports, reminders for overdue reports, and immediate notifications when alerts are triggered-all via SMS. This two-way communication strengthens the surveillance network and maintains engagement.

Alert thresholds must be defined specific to local contexts. When thresholds are exceeded, automated alerts trigger immediate responses. Rapid response teams can investigate at the field level, laboratory samples can be collected for testing, and case investigation forms can be deployed to gather detailed information. The system must be flexible enough to adapt alert criteria as situations evolve.

Integration with laboratory surveillance enhances alert management. Results of case investigations can be collected and appended to alerts, with laboratory users updating results and providing immediate notification back to the field. This creates a complete feedback cycle from signal detection through verification to response.

Rapid deployment during emergencies 🔗

One of EBS’s most valuable features is its ability to be implemented quickly when disasters strike or conflicts disrupt normal surveillance systems. Traditional surveillance infrastructure requires years to build, but EBS can be operational within days.

The EWARS in a box solution 🔗

WHO’s EWARS in a box contains 60 mobile phones, laptops, and a local server to collect, report, and manage disease data. Solar generators and chargers allow operation without reliable electricity. A single kit costs approximately $15,000 and supports surveillance for 50 facilities serving roughly 500,000 people. The system can be rapidly configured and deployed within 48 hours of an emergency being declared.

This portability proved essential in humanitarian crises. Following the 2017 Rohingya refugee crisis in Bangladesh, WHO implemented EWARS across refugee settlements serving over 700,000 people, with training workshops conducted over two weeks. The system successfully detected measles clusters and acute jaundice syndrome outbreaks, enabling targeted vaccination campaigns and rapid investigation.

Bridging surveillance gaps 🔗

During emergencies, normal disease surveillance often breaks down due to damaged infrastructure, displaced populations, and overwhelmed health systems. EBS bridges these gaps by establishing temporary surveillance networks that function independently of traditional systems.

The approach works in diverse emergency contexts. EWARN was first implemented in South Sudan in 1999 after a relapsing fever outbreak response was delayed by six months, resulting in over 2,000 deaths. Since then, similar systems have operated in conflict zones, post-disaster settings, and during disease outbreaks where routine surveillance was insufficient.

Community engagement becomes particularly important during rapid deployment. Community health workers receive training to recognize and report signals of disease, creating a ground-level surveillance network. This community-based component often detects events before they reach health facilities, as community members quickly recognize clusters among neighbors.

Integration with national systems 🔗

While EBS deploys rapidly during emergencies, the goal is eventual integration with national surveillance systems. After the emergency, EWARS should re-integrate back into the national system, strengthening overall surveillance capacity rather than creating parallel structures.

This integration process involves transferring knowledge and tools developed during the emergency to permanent health authorities. Field epidemiology fellows and trainees who operate emergency surveillance systems return to their home countries with enhanced skills, building long-term surveillance capacity across the region.

Moving forward with event-based surveillance 🔗

Event-based surveillance represents a critical evolution in public health surveillance-one that recognizes threats can emerge anywhere and spread rapidly in our interconnected world. By complementing traditional surveillance with rapid detection capabilities, EBS provides the early warning time needed for effective public health response. As climate change and urbanization increase the risk of disease emergence, investing in robust EBS systems becomes not just beneficial, but essential for global health security.

What do you think? How might your community benefit from implementing event-based surveillance for early disease detection? What role could local health workers and community members play in reporting unusual health events in your area?

References

1. https://www.tandfonline.com/doi/full/10.1080/23779497.2020.1848444
2. https://pmc.ncbi.nlm.nih.gov/articles/PMC10712973/
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC7829081/
4. https://www.who.int/emergencies/surveillance/early-warning-alert-and-response-system-ewars
5. https://www.paho.org/en/health-emergencies/health-emergency-information-and-risk-assessment/early-warning-alert-and
6. https://pmc.ncbi.nlm.nih.gov/articles/PMC6199978/
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC5711309/