Health Commissioner On How Antibody Tests Fit Into State Strategy
To date, Vermont has maintained relatively encouraging numbers when it comes to containing the spread of the new coronavirus. At the COVID-19 press conference on Monday, June 15, Vermont Health Commissioner Dr. Mark Levine spoke briefly about the current state of serology testing, or antibody testing.VPR's Mitch Wertlieb spoke with Dr. Levine about how serology tests fit into state strategy, and how they're different from the more prevalent nasal swab tests that test for the virus itself. Their interview has been edited and condensed for clarity.
Dr. Mark Levine: The nasal swab test known as a PCR test uses secretions from the nose to look for active virus. So, it's for someone who's actively infected with the virus. The serology testing is actually a blood test. Most of them require blood draw. But there are some that can be done with just a finger prick. That testing is to look to see if you have antibodies to the SARS-CoV-2 virus, which is the one that causes COVID-19.
[We'd want to find the antibodies] if we knew that having those antibodies was protective for your entire future and that you'd never get this virus again or ever. You know, many people who might have never had symptoms would be interested to know if they had the virus based on the antibody test. But that doesn't mean there's a guarantee that they wouldn't get it another go round. We just don't know that right now.
Mitch Wertlieb: So, Dr. Levine, at the most recent [press conference], you were talking about serology testing and without asking you to reiterate everything you said then, the short version is that the relatively low rate of virus circulating in the population here in Vermont and the limited accuracy of the tests means the positive predictive value of those tests ends up being pretty low. Is that right?
Right. So it's always good to have an accurate test if you're looking for some condition. However, you have to know when you're doing a test like that, how it will perform in the population that you're actually testing. How likely is someone in that population to actually have a positive test? So if you know that the prevalence of the virus in Vermont is only a couple percent versus in a place like New York City, which is just coming out of a major, major problem with COVID-19 and had up to 20 percent of their population exposed to the virus, [when you consider] the likelihood of a positive test in New York City versus a positive test in Vermont, where there may only be a couple percent prevalence of the condition, more often than not, a positive test would be a false positive. In Vermont, when you do the math, three out of every four positive results would actually be a false positive.
Well, given that, is there a place for serology testing in the state public health strategy?
Yeah, I don't want to just throw it out because of this. It's just this helps us understand what we get when we use a strategy like that. But I do think there is a place for serology testing in a state's public health
"In Vermont, when you do the math, three out of every four positive results [for serology tests] would actually be a false positive." - Health Commissioner Mark Levine
strategy. We call that a zero prevalence study, looking to see how many people in a population might have had antibodies formed against the virus. And as, of course, a state's experience with the virus gets greater and greater, meaning more people in the population have been infected, then it's more likely that the positive results will be true positives. The problem right now is that we're really very much laser focused on the other type of testing with all of our pop-ups and with all of our strategically targeted populations who are highly vulnerable. We really do want to focus on that kind of testing so that we can identify disease, contain it, [so] most of the population won't be impacted by it. And I would consider doing the serology testing to be a lower priority at this time, even though it still would have some inherent value.
I know that your focus is on Vermont as it should be, but obviously you're reading the news. You're listening. You're you're watching and seeing everything everyone else is. So you know that in Florida, for example, where the state has reopened fairly aggressively, we have seen a spike in numbers. But what we're hearing from the governor there is he's saying, well, you're seeing more numbers because we're testing more. What's your view on that?
That is a partial explanation, but that's not the full explanation. To get the kinds of numbers that they're having in Florida right now isn't just testing a lot of people, because when you really expand your testing, you start testing a lot of asymptomatic people, and some of them will have the virus. But what we've learned in Vermont is the majority of our testing of asymptomatic people does not yield huge results. So the kind of numbers they're getting in Florida have to also be explained by an uptick in real cases.
The reason I ask is because one thing that Vermont does have in common with Florida, even though they have a much bigger population, is that there is this commonality of a population of a lot of elderly people who are most at risk for COVID-19.
Precisely. And that's why we're so focused on protecting those vulnerable populations in aggregate. When you look at the well over 200 longterm care facilities, 38 of which are nursing homes, I believe we've had a very, very small minority of those institutions and facilities actually become infected.
Finally, Dr. Levine, I know the governor has not mandated that the public wear masks when out in public. But your view on wearing a mask?
You know, we're trying to simplify our message as much as possible. Four key points: Stay home. If you're sick, do the appropriate handwashing and hygiene and wear a facial covering and make sure you physically distance. I think Vermonters really have taken those to heart, and that's really been wonderful.