Public Health Surveillance: Immunity, Testing, and Contact Tracing

Wagner: Because public health has been functioning at such a high level in the United States, I think there's been a lack of understanding of what's the importance of public health. So I’m hoping through this outbreak there's a renewed interest in the American population in public health. I hope more people realize that what we're doing is really important. I hope there's more investments so that when the next outbreak happens, if that's in five years, if that's in 10 years, if that's in 20 years, that we have more infrastructure in place, that may be at the beginning we can even do more contact tracing, that we have a better of understanding of what sort of measures need to come into place to prevent the spread of disease. So maybe in the future when we respond to something like COVID-19, we'll respond even more efficiently and it won't get to the point which is already gone with this current outbreak.

Speaker 1: Hello and welcome to Population Healthy, a podcast from the University of Michigan School of Public Health. This episode is part of a series of special editions of our podcast focusing on the ongoing coronavirus pandemic.

Long before we could sequence viruses’ genome in a matter of weeks, we used public health tactics like contact tracing to sort out the movement of a disease in the population. Contact tracing is one of the traditional tools of epidemiologists and it works like this: 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 a Research Assistant Professor of Epidemiology at the University of Michigan School of Public Health. We asked him to cover some basics on how we monitor a disease outbreak. From how we test for it, to how long we might have immunity to it after an infection.

Wagner: There's a lot we don't know about how the immune system responds to coronavirus. That's because this particular strain of SARS-CoV-2 which causes COVID-19, it's only been circulating for three to four months and only about two 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 think and we have pretty good evidence that once you get infected with measles you are protected all of your life, but measles is a devastating disease. So fortunately, we have a vaccine and just like a natural infection, the vaccine should work your entire life.

So measles is an example where not only is the vaccine or the natural infection, it's gonna result and pretty much a life-long immune response but also even though there are some genetic differences between measles found in different parts of the world, if you have an immune response to one strain of measles you're gonna be protected against other strains as well. By that we mean, there's one serotype, one type of immune response to measles virus.

However, if we look at something like influenza, it gets much more complicated. For influenza there's a lot of different serotypes. There's a lot of different strains of influenza that are out there. People know about influenza A, B. There's also influenza C. There's different subtypes. The ones that are circulating in recent years, each H1N1. There's H3N2. And what that means is that in a given year you could be infected with H1N1 one month, get over that, but then get infected with H3N2 and I get over that and then get infected with influenza B.

Your immune response would be different to each of those because your body just thinks they’re entirely different things. They're related, they're all influenza viruses, but they're sufficiently different enough that your body just thinks they're different.

The other thing about influenza is that it does mutate relatively rapidly so that even if you get an H1N1 infection in one year, in the next year H1N1 may have mutated enough that you won't be protected anymore. And that's also why the strains that we put in the vaccine change each year. So we could actually have H1N1 strains, and we really have since 2009-2010. We've had series of H1N1 in the vaccine but each year they're a bit different and that's because we're following the evolution of the H1N1 virus.

And so if we look at coronavirus again, we really only have about two to three months of high quality data for COVID-19 so we really only know about immune response within this two or three-month period. So it'll take us a year to figure out if we have immunity which lasts a year long. It'll take us two years to figure out if we have immunity that can last two years long. But we can look at what different strains of coronavirus are like. So, people have done studies in the past, not on these pandemic strains of coronavirus, but the strains which cause the common cold, and have looked at how long the immune response lasts for those, and unfortunately, it doesn't seem to last a long time. I would say in the year range, even less than a year, within that period, people will lose immunity even to the same strain of coronavirus. 

Speaker 1: So this means for now, we really are waiting for the vaccine before any sort of full return to normal. But as we return to some regular activities in the coming weeks and months, we'll need to be ready for what robust public health surveillance looks like. And if you haven't already been tested for COVID-19, you might be wondering what that looks like.

Wagner: There's two different ways of testing for a virus or for bacteria in an individual patient. You can actually try to find the genetic material from that virus or the bacteria. So for coronavirus what they do is they stick what looks like a really long q-tip into your nose. They swirl it around a bit and then they try to see if on that q-tip, is there any viral genetic material. And so they run tests, they're called PCR tests. So that's one way of identifying if the virus is replicating within the human host.

Another thing you can do is look at the immune response. Do they have antibodies within their bloodstream against this virus? And so in the United States, we've only had these antibody tests come on to the market relatively recently, so we've mostly been using PCR testing instead of these antibody tests. But there now are antibody tests which are available, that's just getting a sample of blood. So, you'd have somebody do a venipuncture, collect a sample, and then run some sort of antibody tests in a laboratory.

It takes a while to develop an antibody response, so it might take five or six days after an individual is infected for us to be able to really detect an antibody response within the person, whereas we may more quickly be able to find the viral genetic material. We also need to consider how accurate these tests are. So how many false positives, how many false negatives are there? And those vary across these different tests and there's a number of different antibody tests which will be coming on to the market.

Some people are interested in this idea of could we put together an immunity passport where somebody is shown to have had immunity towards COVID-19, could they more quickly return to work or could they be excluded from these social distancing measures? I think that's a bit harder problem to tackle, because if there are some false positives from these immunity tests, then it could be that we're giving people immunity passports, who actually are not immune to the disease. And who maybe will engage in riskier behaviors and then maybe become a Typhoid Mary and be spreading disease around inadvertently.

Speaker 1: What's more likely that once the current outbreaks subsides and the overall caseload is lower, instead of so-called immunity passports, we'll stick with more traditional modes of public health surveillance, like contact tracing, until a vaccine is developed.

Wagner: Contact tracing is a core public health function, and it's something which epidemiologists at the State Health Departments, at local health departments, have been very busy doing over the past couple of months in regards to this COVID-19 outbreak. And basically what it means is that we try to very quickly test individuals to see if they have coronavirus.

So say somebody comes into the hospital, they test positive for COVID-19. What we will then do is to ask them, what have they been doing in the past few days. And so for something like coronavirus which can be transmitted asymptomatic, and it can be transmitted before you have any symptoms, ideally, we should be asking them any contacts they’ve had with any human being up to four days prior to any symptoms. So that could be a really long period. You're asking somebody what did they do in the past week, week and a half, two weeks. So this could be like what neighbors did they have contact with? Obviously anybody in their household would be part of this. What stores did they go to? Did they visit anybody?

And then the epidemiologist will have to contact all of those stores, all of those people. So it's a lot of busy work. It's sitting in front of a computer and using a telephone to call a lot of different people. What we'll try to do then is to make sure that all of those individuals are quarantined. So not only are we isolating the index case, but we're trying to quarantine all those people who had exposure with that index case in the past few days.

Unfortunately right now we are just so overwhelmed with the number of cases in Michigan and across the United States, that we really can't have effective contact tracing in place. Like we're not gonna be able for all of the hundreds of cases in Michigan, to call up everyone that they have contacted. That's too much work. But hopefully we can get to a point in a month or two where if the case count goes down enough, we can really quickly get to these cases, isolate them, and make sure that all of their contacts are quarantined.

But that requires two things: one, it requires us to have enough tests that anybody presenting with coronavirus symptoms can be rapidly tested, it requires us to have the manpower to make all of these calls, and ideally it would require us to have some sort of social safety net so that if somebody is called by the health department and says, "hey you need to stay at home for the next week or two”, that individual isn't going to be too economically affected. But unfortunately, I think all of those points are not the case right now.

Speaker 1: If getting that quarantine phone call, sounds a bit like getting that jury duty letter, you're not too far from the mark. Because despite the public service you're offering with self quarantining or with serving on a jury, these duties for many Americans have a significant impact on our ability to work and to support a family financially.

Wagner: I think that's the huge thing, too. If we had more comprehensive sick leave policies, that has to be a key component of this. Otherwise, why would people adhere to it?

Speaker 1: This has been a special edition of Population Healthy, a podcast from the University of Michigan School of Public Health. During the ongoing coronavirus pandemic, we’ll work to bring you analysis from our community of experts to help you understand what this public health crisis means for you. To stay up-to-date in between special edition episodes, be sure to check out our website publichealth.umich.edu, subscribe to our Population Healthy newsletter at publichealth.umich.edu/news/newsletter and follow us on Twitter, Instagram, and Facebook @umichsph.