Even people who have been vaccinated several times can become infected with the corona virus. Scientists have now determined antibody values ​​from which the risk of such a breakthrough infection increases. The results could form the basis for a blood test.

Vaccination against the coronavirus protects against Covid-19. How well, however, depends on the strength of the immune response that the body develops against the virus – and this differs from person to person. This is shown by the largest German vaccination study on Covid-19, which a research team from the medical faculty has been conducting for more than a year at Essen University Hospital. The scientists have now determined values ​​from which the risk of infection increases despite vaccination.

Under the direction of the Institute for Pharmacogenetics and the Institute for Virology, more than 2500 employees of the University Medicine Essen were included in this study from spring 2021 and continuously examined. Blood samples were taken regularly after the first, second and third vaccination and the amount of antibodies against Sars-CoV-2, the so-called antibody titer, was determined. In addition, the participants answered questions about their state of health and whether corona infections occurred despite vaccination.

The research team has now published its first study results in the renowned journal Frontiers in Immunology, which includes the data from 1391 participants. In the period from the end of November 2021 to the beginning of March 2022, 102 people, or seven percent, became infected with the omicron variant of Sars-CoV-2 despite a booster vaccination. Most infections occurred in the private sphere and not at the workplace in the hospital.

“The good thing about the news is,” explains Winfried Siffert from the Institute for Pharmacogenetics, “that the disease lasted only a short time and was mild in all those infected, similar to a cold. No one needed hospital treatment. So we see confirmation that after a booster vaccination you are protected from a severe course despite the infection.”

The research team also looked into who was affected by breakthrough infection. “Age, gender, previous illnesses or the like played no role here,” says Ulf Dittmer from the Institute of Virology. “However, infected people had lower antibody titers compared to non-infected people, so they responded less well to the vaccination – why is the subject of further investigations.”

The concentration of antibodies in the blood is usually given in the unit BAU, short for Binding Antibody Units. Specifically, the scientists found that the antibody value was average

“Study participants with an anti-spike antibody level of 2816.0 BAU/mL or less had a two-fold increased risk of breakthrough infection compared to subjects with antibody levels above this threshold,” the researchers continue.

In addition, the scientists used a neutralization test (sVNTs) to investigate how well the antibodies present could inhibit the omicron variant. This value is given as a percentage. Another difference was noticeable here: the blood serum of infected people was able to neutralize the virus variant omicron much more poorly than was the case with non-infected people. In addition to the lower number of antibodies, the reason probably lies in the nature of the antibodies. The Essen team also wants to examine this point more closely in the future.

The final result was therefore:


had a tenfold increased risk of developing an omicron infection one month after the booster vaccination despite the booster vaccination. On the other hand, the risk of developing an omicron breakthrough infection was increased, regardless of vaccination schedule, gender, body mass index, smoking status, or previous illnesses.

The researchers say their study results could help identify people at higher risk of breakthrough infections. This could be done using routine antibody determinations and sVNTs, i.e. a blood test. If the values ​​are too low, the vaccination protection could then be refreshed again.

People who have been vaccinated can already have their antibody levels checked in certain test centers or medical practices. However, they do not offer a uniform method of combining these with sVNTs.

However, the vaccines were not only developed to prevent infections, but also to protect people from a severe course. They continue to do so for most people.