Antibody tests can determine if people have already had coronavirus. In the future, they can tell us which people are immune – thereby helping us return to life as normal.
Antibodies are microscopic proteins that help your body fight dangerous infections like COVID-19. They are the tool the immune system uses to identify pathogens. They set defense cells onto the pathogens which can then render them harmless. The presence of antibodies in your blood can indicate if you have already been infected with a specific pathogen – in this case the coronavirus SARS-CoV-2.
In this article we answer the important questions about antibody tests. What are antibodies? Which ones are active against COVID-19? What are antibody tests good for, and how do they help us develop herd immunity?
Note: The latest information and official recommendations can be found additionally on the NHS website. On our Health Info page we have also provided you with important facts and recommendations for coronavirus and COVID-19.
What are antibody tests good for?
Why are some researchers pinning their hopes on antibody tests?
With an antibody test like the cerascreen® Coronavirus Antibody Test you can find out if you have already had the virus. Many people are unaware that they have had the virus. It commonly presents with mild symptoms that appear to be cold symptoms, or there are no symptoms whatsoever. 
Once I have had the virus, am I immune?
It is still difficult to say if people who have antibodies against the virus are actually immune. Reports from China and South Korea seem to show that the virus can be active more than once, but experts are blaming this on the inconsistencies in testing. 
Experts like the virologist Christian Drosten believe that this virus behaves like other cold viruses, which would mean that one would develop immunity after having had the virus, which could last even up to 2 or 3 years. 
As soon as it has been scientifically proven, there could be new possibilities for people with immunity. When one is no longer ill and cannot infect others, they could be allowed to socialise again and return to work. In the future, the government may in some cases remove restrictions for those with a positive antibody test. A so-called “proof of immunity” ID is being discussed.
When should I use an antibody test or a PCR test?
Antibody tests cannot be used to reliably establish if you are currently infected with COVID-19. This is because the immune system only produced antibodies in the end stages of an infection, and therefore such a test is not useful to detect infection in its early stages.
To diagnose an acute infection, a PCR Test can detect the genetic material of the virus in a DNA sample.
What is herd immunity?
Herd immunity occurs when enough people have been infected and are now immune, so that the virus can no longer spread exponentially. As regarding COVID-19, one could say that this is the point at which the pandemic no longer requires safety guidelines and social distancing to be controlled. In order to achieve herd immunity, according to experts around 60 to 70 percent of the population needs to carry antibodies. How far along we are in this process needs to be checked with antibody tests. .
How will we achieve herd immunity?
The important question is, how quickly we will achieve herd immunity? If the pandemic spreads at a tempo that our health system can handle, it could take years for enough people to become infected. According to current research estimates (as of 24 April 2020), only two to three percent of people have developed immunity. That’s still a long way to go. , 
If too many people become infected in a short period of time, than the health system will be unable to provide adequate care. There will be shortages of hospital beds, medical staff, respirators – which can lead to more deaths. To speed up the process of herd immunity, experts are hoping to develop a vaccine, which actually produces immunity. But according to estimates, we will not have a safe vaccine until the beginning of 2021 at the earliest.
What are antibodies?
Antibodies are proteins that the immune system uses to defend the body from pathogens. In medical terminology, these are called immunoglobulines.
Did you know? An antigen is a substance on the surface of a pathogen. Antibodies react with these substances, or more exactly, they “bind” to it. When an antibody is bound to a pathogen, then the defense cells can recognise it as an invader and try to eradicate it.
What different kinds of antibodies are there?
There are different classes of antibodies that are responsible for fighting different types of pathogens. The most important classes are: 
- IgA antibodies protect from different types of pathogens. They are present in different mucous membranes, such as the gut, the airways, and the eyes.
- IgE antibodies fight parasitic worms. They also are responsible for allergies.
- IgM antibodies are the first line of defense for the immune system. They are active in the early acute phase of an infection.
- IgG antibodies are the rearguard of the body’s defenses. They are first produced two to three weeks after an infection has passed. They remain in the body for a long time and establish immunity.
Which antibodies are active against COVID-19?
The antibody classes IgA, IgM, and IgG are usually active against viruses. Studies show this is also true for the novel coronavirus SARS-CoV-2.
In the course of an infection with coronavirus, the body begins little by little to produce antibodies. In the early stages, there are few to none. Then the IgA and IgM antibodies arrive. In later stages – sometimes even during the course of the disease – the IgG antibodies arrive. The IgG antibodies are more specific, they target the specific virus. They also stay in the blood much longer, which is why it is better to test for these in a COVID-19 antibody test.
How long do antibodies stay in the blood, and how accurately can they be detected?
With others of the coronavirus family, IgG antibodies can sometimes be detected up to three years later. The IgG antibodies for other virus families like measles remain for the rest of your life. At this point it cannot be said exactly how long they stay for SARS-CoV-2, as the long term studies do not yet exist.
As they fit perfectly and remain a long time, IgG antibodies are a reliable measure to indicate if you have had a certain disease or have developed immunity to a pathogen. 
Good to know: you have certainly read the term “novel coronavirus” frequently. The SARS-CoV-2 virus is namely the first of its kind. There are several members of the coronavirus family, like the SARS-CoV-1, which causes the disease SARS, MERS, and the common cold. SARS-CoV-2 is very different from the others in the group, which is why our immune systems are not prepared for it. 
Which antibodies should we test for?
IgM and IgA antibodies remain in the blood for a few weeks after an illness. If the infection is a while back, they may already have disappeared. Additionally, these antibodies are not designed specifically for the virus. The probability that a test confuses these with antibodies for a different corona cold virus is fairly high. In contrast, IgG antibodies remain in the blood for far longer, perhaps even for years. IgG tests therefore provide a more accurate result, because they can distinguish better between the different coronaviruses. Experts therefore consider IgG measurements the most diagnostically relevant – when they are taken no earlier than two weeks after the beginning of the illness. In a chinese study, IgG and IgM tests could not return positive more than ten days after the start of an illness, as before the antibodies cannot be accurately detected. After 35 days IgM antibodies can often no longer be detected. 
How accurate are antibody tests?
IgG tests can provide very accurate results. This however is only the case when they are performed long enough after the infection started. Normally the antibodies can be detected around two weeks after the infection. There are two factors that describe the accuracy of such tests: specificity and sensitivity. Both are measured as percentages and indicate how high the probability is that a result is correct. Specificity tells us how accurate a negative result is, so if a person was healthy. A high specificity means there is a low risk of a false positive.Sensitivity indicates how accurate the test measures if a person is sick, so how reliable a positive result is. A high sensitivity means there is a low risk of a false negative. 
Specificity of antibody tests for COVID-19
The standard for IgG antibody tests for coronavirus is the well-established ELISA analysis. Many labs report a specificity for their ELISA COVID-19 tests of 98 to 99 percent. Otherwise stated, the risk of getting a positive result when you actually don’t have antibodies is only one to two percent. The accuracy of the result depends on when the sample was taken. Some labs even report a specificity of 100 percent if the sample was collected at least 20 days after infection. 
How does ELISA work? The blood sample is combined with the virus on a special lab plate. Later an antibody binding enzyme is added. If the right antibodies were present in the blood sample, then an entity consisting of the virus, antibody, and enzyme is created. Then a substance is mixed in which the enzyme acts upon, creating a color change. The degree of color change indicates the concentration of antibodies.
Scientists have another tool to analyse antibodies in order to control for mistakes. If a blood sample tests positive for antibodies, the sample can be tested again in another lab. This is called a neutralisation test and is meant to ensure no mistake was made in the analysis and it is an accurate result. 
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