Surveillance Testing: Gathering the Data on COVID-19

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Q&A with Emily Martin

Associate Professor of Epidemiology

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Right now, public health professionals and researchers around the globe are gathering samples of the new coronavirus. The goal is to better understand how COVID-19, the disease caused by the virus, is spreading and how effective our prevention strategies are. These testing processes are called surveillance testing. The purpose of this type of testing is not necessarily to diagnose individuals but to collect data that allows health systems to understand how the outbreak currently looks and plan for the coming weeks and months. Michigan Public Health experts are now assisting in conducting surveillance testing domestically and internationally in places like New Zealand.

Emily Martin is an associate professor of epidemiology at the University of Michigan School of Public Health and an expert in viral respiratory illnesses. She explains what surveillance testing is and how it can help us in the process of slowing the spread of this virus.

What does surveillance testing mean when it comes to stopping a pandemic? When does it begin?

The purpose of surveillance tests is to monitor the current state of the epidemic. The best way to think about this is how it's currently used for influenza: In this process the CDC will send reports out to say, “Influenza season has started, it's very high, it's very severe, this year, or starting to end.” They can also use this testing data to figure out how useful our tools are. For example, how useful is the vaccine? Or, how useful are antivirals in an urgent situation? Those same systems can be deployed to monitor COVID-19 infections in the same way and we can use those to monitor things like whether it is moving into new areas, whether it is affecting some groups of people more than other groups of people, or whether it is going up or going down.

It’s important to understand whatt surveillance doesn't mean. It doesn’t mean that everybody gets tested. Instead, surveillance means that we test the right samples of the population in a way that allows us to make inferences as efficiently as possible. It doesn't mean that we test everybody because it’s important that we strike this balance between getting the information we need and saving enough tests for clinical care.

Surveillance testing as a priority is second to clinical care. In the initial days of an outbreak, as a virus moves into an area, the testing for that virus will often be centralized at the health department, and the goal is to define when the virus is here. Once that's established, then testing focuses on supporting clinical care and making sure the patients who are most severe are getting taken care of. And once that is under control, then we move onto this surveillance phase where we try to both continue taking care of people who need clinical care while also defining the epidemic as it rises and falls to be able to inform public health. That surveillance system is one that will come into play as the acute situation gets under control.

Case counts are reported every day and seem to be getting higher and higher. In Michigan, we see doubling of these numbers nearly every day. How should we interpret this often startling information?

Case counts are very useful in the early days of an epidemic because they help give us a sense of whether the virus is in a population or not and in which populations we’re seeing the virus impact.

As testing ramps up, it's natural to see case counts explode. So we're going to see case counts that rise and rise, but it doesn't mean that the virus is spreading faster and faster. What it means is that we're doing a better job of testing and capturing and we're learning more about the size of the epidemic. So case counts in this phase of an epidemic can be quite frightening, but really it means that we're learning more and not that the situation on the ground is substantially changing. It takes some caution to be able to watch those numbers rise and remember that our personal level of risk hasn't actually changed a whole lot from week to week.

As we get surveillance systems in place, case counts get converted to incidents where we start talking about the number of people per 100,000 who get infected. We’re then talking about the prevalence of infection or how common the infection is in the population. You usually hear about influenza monitoring in this way, in terms of rate or prevalence. For example, you might hear that one out of 10 people will be infected with influenza this year.

Right now, it's important to remember that we’re in what we call the "exponential phase” of an epidemic, when an epidemic is growing and growing and it hasn't started to turn a corner yet. You may hear people speculating about numbers. I've heard numbers anywhere from 20% to 40% to 60% of people will eventually be infected by coronavirus. It is very difficult to estimate that number because we don't know if the corner is turning now, a week from now, or a month from now, and we don't know how many people will be infected when we start to turn the corner. Without knowing when that turn will occur, it's very hard to extrapolate out. Right now, experts are making their best guess because they want to be as prepared as possible for the worst-case scenario so hospitals are ready and we have the necessary supplies on hand. But, right now, there's a lot of speculation that goes into deriving those numbers.

We now know the outbreak started in China, where it spread quickly. Other countries that experienced the outbreak soon after included South Korea and Italy. In the last month, we've seen a huge divergence in the state of the epidemic in those countries. South Korea seems to be faring far better than Italy when it comes to containment. Why is that?

They’re testing a ton of people in South Korea and those tests are being used to identify who needs to isolate carefully to not infect other people.

But there are a couple of key differences between the US and South Korea. For one, the US is larger than South Korea. There are a lot more people and we are spread out a lot further geographically than South Korea. Additionally, our infrastructure is spread out and our public health infrastructure is not as dense or as easily activated in one local area. Once you find a positive test, you need to do a lot of communication with the health department to find out who the contacts are and to try to spread people out so they don't infect each other. It's very resource-intensive for a country as large as the US. 

What South Korea has done is amazing and I think it's a model. It's a goal that people should look to, but it's a challenge to implement that in other settings.

So, how can other countries respond better to the virus if they are not able to implement the same testing procedures as South Korea?

The data is very clear that responding early, even when it feels like you're responding to something that may not be a big deal, is what works. If you’re initiating a response and trying to get the public's attention but people aren't really paying attention because they don't think it's a big deal and it feels like you're overreacting, that usually means that you're doing the right thing at the right time. If you wait until there is a critical need, then public health-style interventions—interventions that aren't vaccines or new drugs or things like that but rather the kinds of things that we have control of in our day-to-day life, like social distancing—won’t make much of a difference. Those are the things that have to be implemented before the entire population is going to realize this is a crisis.