Yay! Testing Antibody Positive

Everyone is scrambling for ways to re-open the economy. One of the most popular ideas is also a simple one: If someone was infected with Covid-19—and they recovered—it should be safe for them to go back to work. Cities and countries around the world are considering giving ‘immunity passports’ to people who test positive for antibodies to SARS-CoV-2 (the official name of the new coronavirus). Chile has already started to issue them. The approach is practical, but will it work.

Was that cold I had really Covid-19?

Since more than 80% of people who are exposed to SARS-CoV-2 likely never end up in a hospital, the only way to know if they were ever infected is through antibody testing. Antibody tests, also called serology tests, use a drop of blood instead of the nasal swabs that are used to test for the virus. When a person tests positive for antibodies to SARS-CoV-2, it shows that their body has tried to fight the infection. If a person has never been exposed, they can never develop the antibodies. Once a person has Covid-19, it usually takes 10 – 14 days before antibodies appear. So far, so good.

SARS-CoV-2 serology tests are more available—and easier to use—than the tests for the virus, which are in short supply. I don’t know about you, but my inbox is full of offers for at-home serology testing kits. According to the World Health Organization, there are already 275 SARS-CoV-2 serology tests that they know about. Even though there seems to be tons of choice, we still don’t know how well these serology tests work. It turns out that accurate and reliable serology tests are pretty hard to develop.

False positive serology test results

There are two ways that a serology test can go wrong. One is the test itself can be inaccurate. The other is the interpretation of the results.

Scientists have begun testing the accuracy of the most popular serology tests. These are called validation studies. In a recent study, one of the tests found that 14% of the test results were false positives. When someone gets a false positive test result, they believe that they have the antibodies—when they really don’t. If one million people in the US were tested using this particular serology, up to 140,000 people could have a false positive test result. I admit this example greatly oversimplifies the math, but a false positive test would still result in a lot of people thinking they’re protected from getting infected, when they’re not.

Then comes interpretation. In order to correctly interpret a serology test for an individual, the background level of infection in the community must be known. What?!? That’s something we don’t know because not enough virus tests have been done. Right now, serology tests throw light on the extent of SARS-CoV-2 spread in the community, (i.e., the cases that were missed because they were not tested for the virus). Results from serology studies are estimates and are presented with a range of possibilities: it could be as low as this number or as high as this number. That’s not possible for an antibody test on an individual. The vast majority of available serology tests aren’t accurate enough to say with certainty that a person has the antibodies.

Antibodies and Immunity

The fact that SARS-CoV-2 hasn’t been around long enough for people to be infected, recover and be re-infected has led some to suggest that antibodies to Covid-19 may not cause immunity. That’s just plain wrong. Absolutely there will be immunity. But (there is always a but) we do know something about immunity and coronaviruses. It is not black and white.

There are six other human coronaviruses. Four of them are leading causes of the common cold. The other two—SARS and MERS—each caused their own international public health emergencies. If SARS-CoV-2 acts anything like the other coronaviruses (and there is no reason it shouldn’t), immunity will last only a year or two. That’s the same amount of time we hope to have a safe and effective vaccine. But, like Indiana Jones in the Raiders of the Lost Ark, it takes all of the information to make the right call. Immunity also depends on the virus ‘dose.’ The sicker you are, the more likely your body will be able to fight off re-infection in the future. That does not bode well for the estimated 50% of people who never have any symptoms but who test positive for SARS-CoV-2 antibodies. These people would also receive ‘immunity passports.’

The good news is that if people get re-infected, they are likely to have a much milder infection. The bad news is that an unknown, but probably large, number of people who test positive for SARS-CoV-2 antibodies are at risk of getting—and transmitting—Covid-19 a second time. It is also likely that people who think they can’t get Covid-19 again, will behave differently. Perhaps they’ll stop wearing masks, stop obsessively washing their hands, go back to working in close quarters, basically they’ll do all of the things that helped the virus spread like wildfire in the first place.

The Bottom Line

Perhaps we should call them ‘antibody passports.’ That’s not quite the same thing as an ‘immunity passport,’ though. Until we have a vaccine or a cure, we will have to continue to bend the curve. Once the number of cases is low enough, we’ll need to rely on the tried and true strategy of epidemic control: test for the virus, isolate the infected, then find, test and quarantine their contacts. The only way to open the economy now is to identify outbreaks before they become hotspots that require lockdowns.

This article was originally published April 30, 2020 here.