Coronavirus: How Antibody Tests Help Understand the Pandemic

When can we finally leave without worries? Go to concerts, travel, meet friends? The answer to this question is as simple as it is sobering: when the pandemic has stopped.

Chancellor Angela Merkel in her press conference on the first steps since the crown closed toned down expectations of a rapid return to everyday life: the situation was “fragile”. To cope with the pandemic, the latest federal and state government decision must focus, among other things, on determining immunity to Sars-CoV-2 in the general population. This information is important for the evaluation of the subsequent course of the pandemic in Germany.

The Covid 19 outbreak is only considered over when the majority of the population is immune to the coronavirus, either because an effective vaccine has been found or because 60 to 70 percent of the population has had an infection and subsequently developed antibodies against the virus. So, as experts call it, there is collective immunity.

But Germany is still a long way from this: An approved vaccine is not expected until next year at the earliest. And early antibody test results show that even herd immunity doesn’t seem to have been achieved. “So far we can see from the results of our antibody tests that around 3.4 percent of the blood samples were positive or borderline for antibodies against Covid-19,” says Thomas Fenner, head of the Dr. Fenner and Kollegen laboratory in Hamburg. . “Extrapolated to specificity, this corresponds to an epidemic of about 1.9 percent.” Hopes that many people have already overcome the virus without symptoms will be weakened.

The private laboratory carries out antibody tests in Germany for almost three weeks, and to date around 1,300 blood samples have been evaluated. “However, we are actually too early to have really meaningful results,” says Fenner. Because so-called IgG antibodies can be tested, they are present in the blood for a long time and in sufficient quantities, at least three weeks after symptoms begin. “The epidemic broke out in Europe in February,” said Fenner. “So we still have to be patient until all of the patients have developed sufficient, and therefore measurable, antibodies.”

Patience is required

Antibody tests do not detect the pathogen itself, but antibodies, also called immunoglobulins, that the body produces in the course of infection to avoid the virus. Therefore, tests cannot detect acute infection, especially at the initial stage: because the immune system only forms antibodies after a while. But the tests work when the infection has already decreased. Therefore, it can be used to determine how many people have already had a coronavirus infection.

“There is no point in testing the entire population for antibodies now,” says Fenner. “If you already have a positive PCR test, you don’t need an antibody test, for example.” Also, manufacturers’ delivery capabilities are still too small – not enough test materials. Theoretically, his lab could run up to 1,400 antibody tests a day, but the reagents were enough for half.

The Fenner laboratory works with test kits from the Euroimmun company based in Lübeck. A spokeswoman evasively responded to the issue of delivery bottlenecks: “We are trying to process as many orders as possible so that laboratories can begin testing. In order to meet high demand, production capacities are being increasing “.

To avoid supply bottlenecks, Fenner now wants to order a second diagnostic manufacturer: Diasorin, an Italian company. “So of course you have the problem that the two test methods can be different and reliable,” says Fenner. “Therefore, we will first analyze a hundred defined blood samples with both tests and see how the results match and come up with the same result.”

The tests are not perfect

Because in addition to the lack of test materials, there is another problem with antibody tests: they are not perfect. As with any medical testing procedure, there are possible errors.

• The Specificity a test procedure indicates the probability that it actually Healthy recognized as healthy in the test. Because it also happens that someone gets a positive result who has not had any infection (“false positive”).

• The Sensitivity indicate in what percentage sick patient The test is really positive. In individual cases, the test shows a negative result for someone infected (“false negative”).

For example, false positives may occur if someone has recently had another coronavirus infection. Several coronaviruses have been circulating as colds for a long time, so false-positive results would be expected more frequently, especially at the end of the cold season. Virologist Christian Drosten of Berlin Charité said in his podcast on NDR that the rate in his laboratory was three to four percent. However, if the result is positive, more tests are done to confirm it.

The accuracy of the various corona antibody tests on the market cannot be accurately answered. Euroimmun, for example, claims that its tests detect long-term antibodies IgG They only form a few weeks after the infection reaches a specificity of 99 percent. Rather, this means that one percent receive information that they have already had a Sars-CoV-2 infection, although this is not the case. However, in the case of a positive antibody test, it is possible to confirm the diagnosis using additional test procedures or to recognize it as a false alarm, as Drosten also describes.

According to the manufacturer, the sensitivity of the Euroimmun test is 100 percent from day 20 after the onset of symptoms. Before that, however, it was only 87.5 percent.

The published summary of the study in Heinsberg, for example, states that the test used has a sensitivity of more than 99 percent, which means that it gives an erroneously negative result in less than one in 100 infected people.

The Euroimmun test can also detect so-called IgA antibodies, which are formed by the body after only a week. According to the manufacturer, the specificity of the test is 90.4 percent, the sensitivity from day 11 after the onset of symptoms is 100 percent. “Testing at this early stage is pointless,” says Fenner. “Because we have the PCR tests for acute infections. But with the antibody tests we want to know who has already had the infection.” Therefore, blood tests are particularly important for nursing staff or nurses, as they work with risk groups and are at increased risk of infection.

The World Health Organization (WHO) currently recommends the use of antibody tests only in the context of studies. From the WHO perspective, testing is important to understand how widely the virus has spread to the population. This makes it easy to determine, among other things, how high the mortality rate from Sars-CoV-2 infections is. In the medical treatment of patients, on the other hand, the benefits of antibody testing are very limited because the tests cannot quickly detect an acute infection.