Corona: Why some people don’t get infected

It is of course possible that you have already had the virus, but undetected and unconfirmed. For example, in the time when tests were barely available. Or when you had symptoms but the tests never worked. It cannot be ruled out that this was due to incorrect sampling or timing. However, there is no definitive answer that explains non-infection. Rather, the key may lie in a combination of several circumstances. “There are a few hypotheses that seem plausible,” said Leif Sander, head of the Infectious Diseases Clinic at the Berlin Charité.

Unnoticed infected?
First of all, it should be borne in mind that a not inconsiderable proportion of cases go largely or completely unnoticed. In a late 2021 review paper in the “Jama Open Network”, the authors summarized that even with confirmed corona infections, about 40 percent had no symptoms at the time of the test. The basis was almost 100 different international studies with data from a total of about 30 million people. Logically, the test frequency plays an important role. Those who were tested less frequently were more likely to miss a very mild or asymptomatic infection. Mild cases were also found on frequent tests.

In addition, genes may also play a role. “There are people who, due to genetic characteristics, are difficult to get infected with, for example, malaria or HIV. To a certain extent, this will also apply to Sars-CoV-2,” says Sander. However, the genetic factors are not fully understood.

Blood group and vaccination protection
As Ulf Dittmer, director of the Virology Institute at the University Hospital of Essen, explains, the genetic makeup of the immune system – the so-called HLA molecules – plays an important role in protecting against Covid-19. And blood types affected not only the severity of the disease, but perhaps also the transmission of Sars-CoV-2. Vaccination protection is probably often underestimated: the level of antibodies in the blood, which can make the body harmless corona viruses, decreases in the period after vaccination. “However, the protection remains significant for months. That also means fewer infections,” says Sander.

Infection with cold viruses
Immune reactions to vaccination also differ from person to person. “If the answer is particularly good, the combination of vaccination and a previous infection with one of the four common cold coronaviruses may also play a role,” notes the Charité professor. Virologist Dittmer says that it is now known that a special subclass of antibodies offers particularly good protection against corona infection. “However, the measurement is complicated, so for now no one knows whether they have these antibodies or not.” Children, according to Sander, have the phenomenon that they generally have a more strongly activated innate immune system, the immune system is often pre-activated, so to speak. In addition, there is the effect that people are generally less susceptible to the next lurking pathogen for a few days immediately after an infection.

Another conceivable factor: In some people, the immune system can throw the virus out of the body very quickly, as Sander says. “In a Swedish study, researchers found specific T cells in people that did not become positive after contact with infected family members – a sign that their immune system has certainly dealt with Sars-CoV-2, even if an infection and also Antibodies to the virus were not always detectable.”