Using Syndromic Surveillance to find the COVID-19 epidemic in front of us and behind us

Written by Helen Perry PhD and Sharon McDonnell MD MPH

What is syndromic surveillance? 

Public health surveillance is the use of data for action.  It is how we can monitor health problems that occur in our communities, states and countries.  Health care providers like many doctors, urgent care centers and hospitals participate in their State’s notifiable disease reporting system. They report important public health diseases like tuberculosis, measles, meningitis, influenza and respiratory viruses.  Then the public health department can follow up and help contain further transmission to the public. 

Surveillance is really a continuum of approaches for using health data for action.  It starts with a very general awareness that a problem is occurring or an outbreak is suspected. This early warning leads to more specific steps for investigating the cluster or unusual cases, obtaining specimens for laboratory confirmation, and planning actions for an appropriate response in the community.  Each of these steps involves looking at the data collected for that step so that we can monitor how – and if – transmission of the illness or event is being controlled and limited.

Syndromic surveillance is part of this early warning surveillance system.   A syndrome is a collection of symptoms called a prodome, or the early stage of an illness before it is diagnosed.  Sometimes a syndrome may not fit into an existing diagnosis, and so health providers keep track of and report that syndrome because it does not yet have a known cause.  Examples of past public health problems detected through syndromic surveillance include opioid overdoses, vaping-associated lung disease, Zika virus disease, and natural disasters.

Syndromic Surveillance and Influenza-like-illness (ILI) as a tool for COVID-19 tracking

A target of syndromic surveillance that we are hearing a lot about in this pandemic is called influenza-like illness or ILI.  ILI as a syndrome is collected routinely as part of the flu surveillance each season.  When a patient presents with fever (100*F or greater), and a cough and/or sore throat and has no other cause for these symptoms, this is an influenza-like illness and is included in the count of cases reported to the health department.  The ILI data can be reviewed to see if there are unusual patterns or clusters that should be further investigated. For example, are more cases coming in this year than the last few years?  Are people in a certain age group unusually affected more than others such as young adults or people 65 and older? While the same viruses that cause influenza do not cause Covid-19, unusual spikes in ILI visits can be an early warning that the unusual activity in respiratory illnesses is due to another cause.

CDC’s Influenza-like Illness Syndromic Network focuses on emergency departments (ED) because this is where many people seek care for these kinds of illnesses.   All 50 states, Puerto Rico, the District of Columbia and the U.S. Virgin Islands participate in reporting ILI to the Network.  By tracking ILI symptoms of patients in EDs – before a diagnosis is confirmed – local, state and national health departments can use ILI surveillance as an early warning system for public health concerns, including detection of novel influenza or novel respiratory viruses.  

In fact, it was a similar syndromic surveillance system in China that picked up the COVID-19 illnesses—set up after SARS – that raised the alarms in December and gave them a jump on the idea that something was happening.

These systems are still important.  In an article from The Hill (3/202020), Dr. Anne Schuchat, the principal deputy director of the Centers for Disease Control and Prevention (CDC) said “We’re looking at our flu syndromic data, our respiratory illness that presents at emergency departments. Across the country there’s a number of areas that are escalating.” (1)

The CDC is using one of its most reliable indicators to provide early hints about where the next epidemics might spring up. A surveillance system designed to detect sudden upticks in patients who report flu-like symptoms at emergency rooms across the country, built over decades into a system that presents data in almost real time, was the first alarm bell that rang in New York. Those who reported flu-like symptoms, it turned out, were instead victims of COVID-19, the disease caused by the coronavirus.”  Here’s an example of that graph (see next page).  It shows the ILI surveillance data from emergency departments between October 2019 and March 17, 2020.  This year’s data is shown with the red line and is compared with the information from the past 3 years.  The large increase over previous years’ ILI visits started in late December and continued for the next several weeks into January and early February.  In March, EDs were seeing an unusual increase in ILI visits.  This increase was unusual because this was a time when the trend would have been historically expected to decrease.

Now, that same surveillance system is flashing red lights in many states, a potential sign that coronavirus patients are already visiting hospitals, even if their symptoms are not severe enough to warrant overnight stays.

“There’s just dozens of places we’re watching,” Schuchat said. “We really need to expect that the whole country’s at risk here, and we have to look across our health care system within each jurisdiction to have them be as strong as possible.” (1)

The ILI system augments the COVID-19 surveillance by helping us to look ahead where things are “happening” in the US particularly when our testing capacity is so low. (2) At the end of this post there is a graphic of the data from NYC showing an unusual rise in ILI cases.

Data from Maine CDC showed a similar increase in ILI cases even before our first confirmed COVID-19 cases. The ILI system may be a tool that helps us look ahead for hotspots as well as to look behind when we are telling the story of this epidemic. For an excellent description of the ILI system in COVID refer to the link by Abe Stanway in the Medium.



(2) Stanway, A. “Real Time COVID-19 tracking”. March 14, 2020.

(3) For more information about the National Syndromic Surveillance Program

(4) For information on influenza surveillance and specific information on CDC’s Influenza-like Illness Surveillance Network (item 2 on the page) that includes all 50 states, Puerto Rico, the District of Columbia and the U.S. Virgin Islands.

Published by sharon Mcdonnell

Medical Epidemiologist and Community Memeber

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