Public Health Surveillance: Immunity, Testing, and Contact Tracing

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Q&A with Abram Wagner

Research Assistant Professor of Epidemiology


Long before we could sequence a virus’s genome in a matter of weeks, we used public health tactics like contact tracing to sort out the movement of a disease in a population. Contact tracing is one of the “traditional” tools of epidemiologists—an epidemiologist calls up dozens, if not hundreds, of people who might have been in contact with an infected person. By tracking all of these interactions, they can understand how a disease is making its way through a population. 

Today, we have more public health surveillance tools at our disposal, but we still have a lot of work to do before we fully understand how this new coronavirus behaves and what it means to have immunity to it.

Abram Wagner is research assistant professor of epidemiology at the University of Michigan School of Public Health. We asked him to cover some basics of how we monitor a disease outbreak—from how we test for it to how long we might have immunity to it after an infection.

What does immunity actually mean? Can I get the same flu strain twice in my lifetime? Could the same type of a coronavirus—for example, one of the kinds that cause the common cold—infect me twice in my lifetime?

There is a lot we don't know about how the immune system responds to coronavirus, and that's because the particular strain of sars-CoV-2 that causes COVID-19 disease has been circulating only for a few months in North America and Europe. But we know a lot about how the immune system responds to other viruses. I think measles is a beautiful example, where we have good evidence that once you get infected with measles you are protected all of your life. Measles is a devastating disease, so fortunately, we have a vaccine, and just like a natural infection, the vaccine should work your entire life. There are some genetic differences between measles strains found in different parts of the world, but if you have an immune response to one strain of measles, you will be protected against other strains.

If we look at something like influenza, it gets more complicated. There are many different strains of influenza out there and our bodies produce a variety of antibodies in response to flu infections. We have not only influenza A, B, and C circulating with a variety of subtypes, but in recent years we also have a lot of H1N1 and H3N2. For immunity, this means that, in a given year, you could be infected with H1N1 one month, get over that, then get infected with H3N2, get over that, then get infected with influenza B. Your immune response would be different to each of those because your body thinks they’re entirely different things. They are related—all are influenza viruses—but they are sufficiently different from one another that your body is able to distinguish them. Influenza also mutates relatively rapidly. If you get an H1N1 infection one year, by the next year the H1N1 strain may have mutated enough that you won't have immunity anymore. Each year we put a variety of strains into the flu vaccine because we are following the evolution of these viruses in our population. Since the 2009–2010 season, we’ve had strains of H1N1 in the vaccine because that is now circulating broadly enough in our population.

With coronavirus, we have only a few months of high-quality data. So we know only about the immune response within this short period.

With coronavirus, we have only a few months of high-quality data. So we know only about the immune response within this short period. It will take us a year to figure out if individuals have immunity that lasts for a year. It'll take us two years to figure out if individuals have immunity that lasts two years. But we can look at what different strains of coronavirus are like. We have done studies in the past on coronavirus strains that cause the common cold and have looked at how long immune responses last for those. Unfortunately, immunity doesn't seem to last a long time.

How do pathogen tests work? As we await a vaccine, we’ll all need to be tested at some point. What would that look like?

The big difference in tests is whether you test for the presence of genomic material—RNA in the case of coronavirus—or whether you test the immune response by looking for the presence of certain antibodies. In an individual patient suspected of having COVID-19, we are first looking for the genetic material of the virus itself. These are called PCR (polymerase chain reaction) tests, and this is where a health care worker inserts a long swab into your nose, swirls it around gently, then uses materials in the rest of the test kit to detect if there is any viral genetic material in your sample.

If their body has already mounted an immune response, then they will have antibodies against this virus in their bloodstream.

If a patient has potentially been exposed to and recovered from a coronavirus, we would then be looking at the immune response. If their body has already mounted an immune response, then they will have antibodies against this virus in their bloodstream. Antibody tests involve taking a blood sample from a vein rather than a mucus swab from the nose. In the US, antibody tests have come on the market only recently. So far, we have used mostly PCR tests.

One thing to note about antibody tests is that it takes a while for the immune system to develop an antibody response—five to six days or longer after an individual is infected. Typically, we can more accurately detect viral genetic material in the early stages of an infection with a nose swab test.

We also need to consider how accurate these tests are. We have some evidence of false positives. How many false negatives seems to vary across different tests and how they are administered in different systems and geographic regions.

What is contact tracing, and what could it look like in Michigan?

Contact tracing is a core public health function. And it is something epidemiologists at state and local health departments have been very busy doing over the past couple of months in regard to this COVID-19 outbreak.

Basically, tracing contacts is a way to combine a positive individual test with rapid social outreach to see how far that person might have spread the disease. Say somebody comes into the hospital and tests positive for COVID-19. We then ask them what they have been doing in the past few days. With coronavirus—which can be transmitted by people with asymptomatic cases and by people who are about to get sick but don’t yet have symptoms—we would be asking them about any contacts they had with any human being up to four days prior to any symptoms. That could be a really long period. You're asking somebody what they did in the past week, week and a half, two weeks. Certainly, anyone in their household would be part of this. Then, what neighbors did they have contact with? What stores did they go to? Did they visit anyone else? Epidemiologists then contact all those people, stores, and so on to let them know of the possible exposure and ask them to self-quarantine for a period of time.

Robust contact tracing will be a key component of returning to work, school, and other aspects of life safely before a vaccine is developed.

This is a lot of basic communication work—sitting in front of a computer and using a phone to call dozens or hundreds of different people. Unfortunately, right now we are so overwhelmed with the number of cases in Michigan and across the US that we can't yet have effective contact tracing in place.

For now, it would be too much work and not provide accurate results. Hopefully, in a month or two, if the case count goes down enough, we can get to these cases more rapidly, isolate them, and make sure all of their contacts are quarantined. This will require two things: One, it requires us to have enough tests that anybody presenting with coronavirus symptoms can be rapidly tested. Two, it requires us to have the personnel to make all of these calls. Robust contact tracing will be a key component of returning to work, school, and other aspects of life safely before a vaccine is developed.

For it to work, doesn’t it also mean that people are complying with the self-quarantine recommendation made by the epidemiologist on the phone?

Yes, certainly. And we need to have some sort of social safety net in place for those who are adversely affected by having to isolate at home. If someone is called by the health department and told they need to stay at home for the next week or two, how will that individual be economically affected?

More comprehensive sick leave policies will be a key component to compliance.

More comprehensive sick leave policies will be a key component to compliance here. Why would someone adhere to it if it is going to mean they can’t pay bills and buy groceries for their family? That quarantine phone call will seem like getting that jury duty letter. While you do have this opportunity to offer public service, such duties for many Americans will have a significant impact on their ability to work and to support a family financially. To make sure people abide by an isolation order, we will need to consider some changes to employment practices, like expanded sick leave offerings.

So, in order to successfully control this pandemic, we need more contact tracing capacity, more and higher-quality testing, and a social safety net for anyone asked to self-quarantine who will lose needed income by doing so.

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