Corona has already caught most people, but some have still not become infected - why?
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A photo has often been sent via WhatsApp in recent weeks: that of a positive corona test, with the subtitle “Now it got me too” or “Wow, there can really be two stripes on it”.
More two-strip photos followed with "still positive" (bad mood emoji) or "second strip only very slightly!" (cheering emoji).
But some people do not yet have a corona test photo collection on their cell phone - because they have never had a positive test result.
It is true that those who have not knowingly become infected in more than two years of the pandemic are slowly becoming a rarity due to the relaxed measures.
Nevertheless, there are those who have even survived full bars or infected roommates without getting infected themselves.
How can that even be?
After all, the mask requirement has fallen almost everywhere and the omicron subvariant BA.2 is more contagious than any other virus line to date.
There is no definitive answer to this question, but there are some possible explanations.
One of them: Many of those who have not yet been infected have simply been lucky.
However, the supposed lucky ones could soon be among the approximately 70,000 new infections that are still reported by the RKI every day.
Some of them may have already been infected without realizing it.
In an overview paper from the end of 2021 in the "Jama Open Network", the authors summarized that around 40 percent of confirmed corona infected people had no signs of illness at the time of the test.
Especially those who rarely test themselves could have overlooked a mild infection.
Nevertheless, many were actually not yet infected and will survive the current corona wave without a positive test result.
In science, some hypotheses are being discussed that could justify this.
Genes and blood groups as possible factors
For example, genes could play a role.
»There are people who, due to genetic characteristics, are difficult to be infected with malaria or HIV, for example.
In certain gradations, there will also be Sars-CoV-2, «says Leif Erik Sander, who heads the clinic for infectiology at the Berlin Charité, according to the German Press Agency.
However, the genetic factors are not fully understood.
Ulf Dittmer, Director of the Virology Institute at Essen University Hospital, also thinks it is possible that the genetic make-up of the immune system - so-called HLA molecules - plays an important role in protection against Covid-19.
In addition, blood groups not only influenced the severity of the disease, but perhaps also the transmission of Sars-CoV-2.
A French study supports this assumption.
The researchers' results suggest that the risk of infection increases when the blood groups of the infected are compatible with each other - for example, when both have the same blood group or the first infected person has blood group 0 and the infected person has blood group A infected person has a blood type that the other person who donates blood cannot tolerate.
This would also explain why people with blood group 0 have the lowest risk of becoming infected.
Blood group 0 is compatible with all other blood groups, but can only accept donations from your own blood group.
According to the results, people with blood group 0 could pass on the virus particularly easily, but the likelihood of them infecting others decreases.
"Because blood type A is more common than blood types B and AB, people with blood type A are less likely to encounter incompatible infected people in a population of Western European descent," the study authors write.
The data analysis probably explains why, according to current knowledge, people with blood group A have a higher risk and people with blood group 0 have a lower risk of Covid-19 than the average population.
Role of T Cells
Another conceivable factor: In some people, the immune system may throw the virus out of the body very quickly.
"In a Swedish study, researchers found specific T cells in people who did not become positive after contact with infected household members," says Sander.
"A sign that their immune system has dealt with Sars-CoV-2, even if an infection and antibodies against the virus were not always detectable."
T cells are able to render infected cells in the body harmless and thus prevent a virus from spreading further.
They provide an immunological memory.
Even years after an infection or vaccination, they can recognize and fight pathogens.
They are less specific than antibodies, which is an advantage with regard to corona.
So if T cells are available in sufficient quantity and quality in people who have already been infected with corona viruses in their lives, these could also protect against symptomatic Covid courses.
Vaccination protection is therefore also underestimated in this context.
The many infections despite vaccinations show that vaccination does not necessarily protect against infection.
This is because the level of antibodies in the blood, which can render harmless corona viruses that have entered the body, fall in the period after the vaccination.
However, vaccination also generates T cells.
And although these do not protect against infection with the virus, they do offer good protection against serious illnesses.
According to Charité Professor Sander, the protection remains significant for months.
Immune responses 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 can also play a role," says Sander.
Virologist Dittmer says that it is now known that a special subclass of antibodies provides particularly good protection against a corona infection.
"However, the measurement is complicated, so for the time being no one will know whether they have these antibodies or not."
Preactivated immune system in children
According to Sander, children also have the phenomenon that they generally have a more strongly activated innate immune system, so to speak, the immune system is often pre-activated.
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.
"One of the reasons for this is the so-called interferons, special antibodies in the mucous membrane, which also reduce the susceptibility to Sars-CoV-2 in the event of contact in the time window."
Anyone who thinks they have been spared so far could either have the infection behind them.
Or have benefited from certain temporary effects, as yet unknown genetic factors and coincidences.
According to Sander, that's by no means a guarantee that those people won't get caught: "The fact that you haven't had Corona before doesn't mean that you're safe for all time.
It can look completely different with a new virus variant or depending on the situation.«
Gisela Gross, dpa/kry