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More than 4,200 new corona infections in Austria on Wednesday: That is about as many as last in November 2020, before Austria disappeared in a six-month lockdown, as fewer and fewer intensive care beds were available. Does my vaccination still protect you, one might ask, and can an antibody test in the laboratory provide the answer?
From a price of around 20 euros, corona antibodies are available in numbers with explanations in English in black and white. In the report, for example, next to the name “Covid 19 IgG Antibodies” the number 1563 of the AU / ml unit, or next to “Internat. WHO Unit Sars-CoV-2” the number “222 BAU / ml” with an addition that translates reads: “From a value of 15 MAU / ml” the result correlates with the neutralization test and indicates the presence of neutralizing antibodies against Sars-Cov-2 “. That is worth something. But what does it mean?
Good question, my own experts say. Because antibody tests are not the same as antibody tests. Not all of them indicate the same thing and not all of them provide information about whether a third vaccination is necessary.
Buffer antibodies are difficult to measure in the blood
“Let’s assume I breathe in a good portion of the highly infectious Delta variant, which is distributed in the upper respiratory tract,” says Lukas Weseslindtner from the Center for Virology at the Medical University of Vienna, and explains: “On the mucous membranes of the respiratory tract of vaccinated people there are concentrations of antibodies that, like a buffer, intercept what is coming.” If the virus concentration for these antibodies is too high, it would lead to infection. At the same time, however, a reserve group of immune cells would step in to weaken the disease. “The antibody tests that are carried out in the blood do not depict this complex process,” says Weseslindtner: “You can test this” stepping-in “in research, but it is time-consuming and therefore not suitable for the masses.”
The direct ability of immunological memory to jump into action is therefore only shown as a correlate in a blood antibody test. “From the antibodies in the blood I can deduce how strong the reserve force is, that is, memory cells of the immune system,” says the virologist. If someone has a lot of neutralizing antibodies in the blood, it can be assumed that they are also active on the mucous membrane in the respiratory tract and that the virus is more difficult to get through against these antibodies.
Undifferentiated measurement of all antibodies
But there is a second problem. Most antibody tests do not measure neutralizing antibodies. “Most of the time, classic Elisa tests are done, the results of which are BAU antibodies (Binding Antibody Units per milliliter of blood serum). These tests measure all antibodies that someone has ever produced against the entire spike protein of Sars-CoV-2, without specific ones to see to it that they protect directly against the disease, “says the virologist Elisabeth Puchhammer-Stöckl, “Scientist of the Year” 2020.
That means: From the BAU per milliliter value it can be concluded whether a vaccination has worked, but not how well the person is protected against Covid-19. According to experts, every person has a different mix of neutralizing and non-neutralizing antibodies. Since BAU / ml only indicates the total mixture, someone with 220 may even be better at warding off Sars-CoV-2 than someone with 500, “apart from the fact that the tests from different manufacturers give different values,” says Weseslindtner.
What the BAU / ml value really means
So what does the value “Internat. WHO Unit Sars-CoV-2 222 BAU / ml” really mean? On the one hand, something good: the body should have an immunological memory that prevents a stay in the intensive care unit. Any further assessment is difficult, however. “The WHO has used a certain blood serum as a standard for the BAU / ml value. However, the tests usually do not measure the WHO standard. It is only converted from the test’s own unit Supermarket, very prone to deviations, “says the virologist. If test-specific units such as the “Covid 19 IgG 1563 AU / ml” mentioned at the outset are specified in addition to the standard unit, this is often not adequately explained in the findings.
So far so confusing. But there is a ray of hope. The real protection rate against infection with Covid-19 is provided by a test for neutralizing antibodies, which may only be carried out in laboratories with security level L3. L3 laboratories are allowed to work with infectious viruses and also let go of them on antibodies. Those who have been tested then know how many antibodies that prevent infection are still present in the blood serum.
It depends on the neutralizing effect
How can such a test be obtained? “We measure in the laboratory whether your antibodies neutralize Sars-CoV-2, on 600 to 800 samples a week,” says Weseslindtner, referring to the possibility of getting on Institute for Virology at the Medical University of Vienna, in the Ages or register for it at the Innsbruck Medical University.
For all other laboratories there is also the option of surrogate neutralization tests. They do not measure antibodies against the spike protein, but the binding of the spike protein to the human cell in order to crack it. It is analyzed whether the antibodies prevent this binding and thus the initial contact between virus and cell.
“We compared these tests, which in principle any laboratory can do, with the neutralization tests, and found that they provide just as good information as to whether someone is protected against various virus variants, for example,” says the expert. However, if you want to reduce the likelihood that the virus will hijack your body, you should get a third vaccination six to eight months after the primary vaccination, because the antibody concentration has been shown to drop even if the vaccination is very successful.
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