Author: Sara Habibipour
As antibody tests become more widespread, there is still a major concern that remains: if you test positive for antibodies, does that mean you are immune to COVID-19?
To answer this question we have to understand what immunity is. Immunity is the state of being resistant to a pathogen. There are two different types of immunity: humoral and cellular.
For the purpose of this article, we'll be focusing on humoral immunity.
Humoral immunity is when a B-cell (a type of immune cell) matures into a plasma cell, and secretes antibodies. These antibodies bind to the virus and destroy it.
The way that we can test for these antibodies is through an IGM blood antibody test. The good thing about these tests are that they are cheap and give results quickly. There are two types of tests that are tested for: IgM and IgG antibodies. IgM antibodies signify a current infection, and peak around days 12-35 after being infected. IgG antibodies are a delayed immune response that show up longer after infection, peaking at around days 17-49. Both of these tests indicate that a patient has been exposed to Sars-Cov-2 (the virus that causes COVID-19) at some point. But, although these antibody tests may signify immunity for other pathogens, we really don't know if a positive antibody test means immunity to COVID-19.
A positive antibody test does not necessarily mean that a patient is immune to the virus because we do not yet know how protective the antibodies are and how long protection will last. Also, false positives can occur when someone who's tested has also been exposed to another coronavirus.
As far as knowing how protective the antibodies are and how long protection will last, simply time will tell. This is a novel virus and not enough research has been done to prove anything. Because Sars-Cov-2 is also a RNA virus, it mutates quickly (making research into certain aspects of the virus more difficult). RNA viruses are readily adapting to environmental conditions and make copies of themselves rapidly. This is the main reason why it's so difficult to create a vaccine for Sars-Cov-2 (and other RNA viruses), and is also the reason why we have to get a flu shot every single year!
A positive antibody test may also not be so reliable because if someone has been exposed to another coronavirus, it is possible that the test will account for the antibodies created as a result to that exposure, and not Sars-Cov-2, leading people to believe that they are safe (or "immune") to the virus when they very much aren't. Coronaviruses are much more common than you think. In fact, the common cold is a coronavirus! The only differentiating factor between Sars-Cov-2 and other coronaviruses are the spike proteins on its surface, therefore the antibodies to these viruses are also very similar.
This is why it is important for antibody tests to keep evolving. We cannot be 100% sure of a positive test until we have a test specific to the spike proteins. This is also why many physicians only consider a positive blood test to be significant only when COVID-19's specific symptoms are present.
A couple months ago, the World Health Organization came out saying that they had an idea for "immunity passports." Essentially, the idea was if you tested positive for Sars-Cov-2 antibodies, you could travel to other countries again. But, as we now know, there is no evidence proving that those who have antibodies for the virus are immune.