Governments around the world are hopeful that antibody tests can tell them which people are protected from reinfection and can safely go back to normal activities
By Dr. Eduardo Gonzales
The idea of providing proof of immunity to allow workers to return to their jobs is being considered in many countries, including Britain and Italy. Officials in other countries like Chile, France, and Germany are already taking this step, too. They depend on something called antibody test, a blood test that determines whether someone has ever been infected with the coronavirus.
We would welcome knowing whether we had antibodies and, of course, would be happy to be able to resume something like normal life. But as with any test, they are not perfect, and there have been problems with their accuracy. Here’s what you need to know.
What are antibodies?
An antibody is a tiny, Y-shaped protein that certain cells of your immune system produce when an antigen, such as a disease-causing microorganism like the SARS-CoV-2 virus, enters your body.
An antibody helps fight off the antigen that triggered its production. But it will not kill or dismember the antigen. It simply binds or attaches to it. Also, an antibody binds only to the antigen that triggered its production. Thus, if you get infected with SARS-CoV-2 virus, your immune cells will start producing antibodies that will bind to SARS-CoV-2 virus only. These antibodies will not bind to any other viruses or antigens.
By binding to antigens, antibodies help contain viruses in several ways. Some viruses lose their capability to enter cells where they can propagate when they have an attached antibody. Some “attached antibodies” activate the complement system—a group of proteins circulating in the blood that can destroy antigens when activated. Also, attached antibodies serve as handles that make it easier for immune cells to grab and gobble up antigens.
What is an antibody test, and how does it work?
The immune cells produce five types of antibodies: immunoglobulin M (IgM), immunoglobulin G (IgG), immunoglobulin A (IgA), immunoglobulin E (IgE), and immunoglobulin D (IgD). There are blood tests that can identify and quantify these antibodies. Most of these existing blood tests measure either IgM or IgG.
On entry of a new antigen, the immune cells produce a lot of IgMs, but IgM production declines rapidly such that significant levels persist in the blood for only up to three months. IgGs, on the other hand, are not produced as abundantly as IgMs, but they remain in circulation for years. Thus, a positive result for IgM suggests that a person is currently or recently infected, while a positive result for IgG and negative result for IgM suggests that the person may have been infected in the past.
As a rule, antibody tests are not used to find out if someone has an active Covid-19 infection. This is because it takes the immune cells a week or more to produce enough antibodies that can be detected by these tests. The test performed on suspected Covid-19 cases is a swab test where a six-inch-long swab is inserted deep into the nose or throat to collect a tissue sample. The swab is then sent to a lab for molecular testing.
One use of Covid-19 antibody tests is to identify donors for convalescent plasma therapy. This procedure involves transfusing seriously ill Covid-19 patients with plasma—the liquid component of blood—from donors to augment their self-generated antibodies. Covid-19 positive patients who have recovered from the disease or are asymptomatic are tested. If they still have high levels of antibodies, they are suitable donors.
Antibody tests are also used to find out how many people in a population have already been infected with Covid-19 but were asymptomatic or came down with mild symptoms only. This will enable public health workers to determine the true infection rate and better predict the likely future course of the outbreak and map out the interventions needed to control it in the given population.
If I have antibodies, I’m immune, right?
Not necessarily. In general, an encounter with a virus confers long term, often lifetime, immunity to the virus. This is true for measles, polio, and a few other viruses. So if you test positive for IgG against Covid-19 you could already be immune to the Covid-19. But, as the WHO recently said, there is currently no evidence yet that people who have Covid-19 antibodies are protected from a second infection. The simple truth is there are still too many unknowns about the nature of the Covid-19, and these include whether a previous encounter with the virus offers some kind of immunity.