Immunity after Covid-19 infection

Are you protected against re-infection with SARS-CoV-2 after an infection? If so, does the protection last and make us immune to the Covid-19 disease? What about the vaccination?

Antibodies are formed (Y-shaped) against the surface of SARS-CoV-2 (right).

© Shotshop / Wolfgang Rieger

Covid-19 has many faces. Some people seem to have no or very mild symptoms; others become so seriously ill that they have to be treated in the intensive care unit. The body's immune defense seems to be similarly different in the case of a SARS-CoV-2 infection. There are currently many international studies that deal with immunity to SARS-CoV-2 - i.e. the protection created by the body against renewed illness. Science is in lively exchange around the world about the latest findings. Some theses are confirmed in some places, and shortly afterwards refuted in others. It is part of everyday scientific life that when researching unknown pathogens, the statements must always be checked and possibly reformulated. Everything we know about SARS-CoV-2 is the result of this ongoing re-evaluation and evaluation and the close collaboration between researchers worldwide.

This article reflects the current state of knowledge as of the date indicated. It is updated regularly according to the latest knowledge.

Which defenses does the body mobilize in the event of a Covid-19 infection?

When infected with SARS-CoV-2, the body's own defense system is activated. This process is similar in many diseases:

If our immune system comes into contact with a pathogen, it begins, among other things, to form antibodies against it. There are five different classes of these antibodies. For the defense against SARS-CoV-2, the classes IgG, IgM and IgA seem to be the most important ones so far. IgM antibodies are formed early by the immune system and indicate a fairly recent infection. After a certain time, a so-called antibody switch takes place, then mainly IgG antibodies are formed. They ensure long-term defense against pathogens. Certain IgA antibodies are mainly found on mucous membranes, such as in the mouth, nose and throat. Since the mucous membranes are the preferred entry point for SARS-CoV-2, these antibodies were also examined more closely in studies. There is evidence that they also play a special role in defending against the virus.

The newly formed antibodies are specific, which means they fit exactly on special binding sites on the surface of the SARS-CoV-2 virus. Together with other cells of the immune system, they can render the virus particles harmless. The disadvantage: the formation of the antibodies takes time. Our immune system first has to get to know the virus and the structures on its surface.

Cytotoxic T cells and their role in Covid-19

Another important role in the fight against Covid-19 is played by cytotoxic (roughly translated as "cell-killing") T cells - sometimes also called T-killer cells. These carry a special protein on their surface and are therefore also referred to as CD8 positive cytotoxic T cells. The T cells belong to the white blood cells and recognize the infected body cells in which the virus is already multiplying in viral diseases. If such an infected cell is recognized, the T killer cells kill it. This means that the infected body's own cell cannot spread the virus any further. Initial studies from Essen and Wuhan indicate that T cells can also play a role in Covid-19. They found that people with low levels of T killer cells had more severe courses of Covid-19. T cells can be reduced by chemotherapy, certain drugs, or other diseases that affect the immune system. But also in old people there are often fewer T cells. Even being very overweight can negatively affect T-cell activity.

How long does the protection last?

The protection created by antibodies lasts for different lengths of time in different diseases. In the case of some diseases, contact with the pathogen, or even parts of it, results in lifelong immunity. In some other diseases, antibody concentrations drop over time and there is no longer adequate protection. Then you can get sick again. After a Covid-19 infection, it is currently unclear how long the antibodies have been present in the blood and whether their concentration is then sufficient for an effective defense against the disease. Since the virus has only been known for a few months, a reliable statement on long-term immunity can only be made in a few years. However, there are already studies that suggest that the antibody concentration in the blood can sometimes drop rapidly. Since it is currently unclear what role the various mechanisms of the immune system play in the SARS-Cov-2 defense, one cannot say what significance the measured decrease in antibodies actually has.

Do you become immune to SARS-CoV-2?

Many antibodies against a pathogen can be an indication that the immune system will quickly recognize it upon renewed contact and thus protect the body against an illness. Therefore, many studies are dedicated to the antibody concentrations in people who have already had Covid-19 disease:

A study by the Lübeck Health Department examined 110 people with Covid-19. Around 30 percent did not have any antibodies against the virus. There are similar studies from Switzerland and China. Here, too, people with mild Covid-19 infections had no or very few antibodies in their blood or only for about 35 weeks. The first German infected people were also examined in Munich to see whether their antibody concentrations had changed. In fact, it was also found in these that, over time, fewer antibodies against SARS-CoV-2 were present. However, these are completely new studies, in some cases so-called pre-publications, which have yet to be scientifically confirmed.

Another previously published study from Essen and Wuhan examined 327 sick people who were treated in a hospital. In 80 percent, active antibodies were still found after six months, which were sufficient to render the virus harmless. So the question arises as to whether the severity of the disease has an influence on antibody formation. Further research must also be carried out to determine whether some infected people actually do not form any antibodies against SARS-CoV-2. According to Professor Ulf Dittmer, Director of the Institute for Virology at the Essen University Hospital, an antibody response against viruses is usually prompted quickly. The amount of antibodies first rises sharply, reaches a peak, then falls again and then stabilizes at a level that can usually still provide protection against a new infection, explains the expert.
In addition, it is not yet clear what amount of antibodies would be sufficient to prevent re-infection. Despite the presence of antibodies, if the amount is too low, another disease could occur. But whether you can really get Covid-19 again, if no or only a few antibodies are detectable in the blood, also has to be investigated. So far, no case has been reliably proven in which a person who had already gone through Covid-19 fell ill again.

Immune system reminder - important for fighting pathogens

Because the T cells could also play a role in the "memory" of the immune system of the known pathogens.
And another aspect is important: SARS-CoV-2 belongs to the large class of coronaviruses. These have been around for a long time and they often cause colds. In some studies, it was therefore examined whether people who have already had infections with other coronaviruses are better protected against the pandemic virus. So far, there is much evidence that the antibodies that were formed against the "harmless" coronaviruses can also have a supportive effect in the defense against SARS-CoV-2. This is also known as a cross-reaction.

It still takes time and studies to clarify all of these questions, because even the very first people infected only got Covdi-19 seven months ago. So far there has not been any reliable evidence of secondary illnesses. In Germany, the Robert Koch Institute coordinates nationwide research on the complex topic in large-scale "antibody studies".