Why antibody testing is needed
Sajeda Amin
Shelter-in-place or lockdown policies come at a high economic and social cost. It is widely acknowledged that testing can provide vital information and should be the key to policies for opening up. But what do we know about testing, and why is it important? This week, coronavirus news in the US has been dominated by new data on testing. Here are my takeaways about the confusing medley of information on testing:
It is important to differentiate between two broad types of tests -- tests to check for the presence of the virus and active infection (antigen test) and antibody tests. An antigen viral test tells us whether we are currently infected by the coronavirus.
Early clinical signs of infection (Covid-19) due to the novel coronavirus can be similar to and confused with another, not-so-novel coronavirus, the common cold or flu. The tests check for the presence of virus in the mucus through a deep nose or throat swab.
In many settings across the globe, testing capacity was limited in the initial days of the pandemic. Now it is more widely available and there are tests that are easy to do but are not uniformly reliable. There is a relatively high level of false negative tests, meaning the test says you do not have the disease when in fact you do.
However, there is a bigger problem with extrapolating from this test to answer the question of “how likely am I to get the disease?” This likelihood depends of a number of factors, some unknown, related to an individual’s frailty, virus strain type, as well as exposure, to name a few. While we do know quite a bit about how exposure to someone who is actively and overtly sick, there is a lot we do not know about the potential for spread by exposure to someone who is not overtly sick.
Here is where antibody tests can help -- these tests answer the question of who has been exposed to the virus and has acquired antibodies. To do so, we look in the immune system to ask if it has seen the virus. By looking for markers that the body has mounted an immune response to the novel coronavirus we deduce that it has been exposed.
When the body engages in a fight with the virus, there will usually be symptoms. However, not everyone has symptoms. Some people do not mount a strong antibody response and tests are not able to differentiate between an effective or neutralizing response versus a binding response that does not reflect resistance to disease.
It is clear, however, that all antibodies indicate exposure. More crucially, individuals who test positive, even if they were not sick themselves, may still have spread the virus. New York State showed the highest levels were in New York City, the epicentre of the disease, where 24% of randomly selected healthy individuals had antibodies to the virus.
Lower rates were detected in rural areas of the state. In other words, the disease is more widespread than the numbers of antigen test positive cases or deaths suggest. Now why is that? One reason is the proportion who test negative even when they are in fact infected, the “false negatives.”
However, a more important reason may be the “silent spreaders” -- the many people who have the infection and can unknowingly spread it because they do not have any symptoms. They may either never show symptoms, ie they are asymptomatic, or they may show symptoms later, ie they are pre-symptomatic. One study in Iceland suggests that nearly half (43%) of all people who tested positive did not have any symptoms.
Now, how are these numbers relevant and what does it mean for the average person? The antibody test is useful for understanding the nature of the disease at a population level. It can also be measured repeatedly to understand contagion patterns at a population level.
Unfortunately, the antibody test is not yet ready to serve as an “immunity passport.” It is not able to tell us with certainty whether a person with antibodies is immune to the disease. There is a lot about this novel virus we do not know.
Until we do, we need keep washing our hands with soap and water, use masks, and maintain distance from others to make sure we do not infect ourselves and others. These behaviours and distancing are our main recourse.
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