Why are there so few moderates

Course of COVID-19Why do some get sick more violently, others hardly?

COVID-19 is a disease with many faces. While most viruses cause clear symptoms, an infection with SARS-CoV-2 can damage the body in a wide variety of ways. According to the Robert Koch Institute (RKI), around 80 percent of the illnesses are mild to moderate, i.e. with at most a slight pneumonia. In 20 percent there is a clinical deterioration with severe pneumonia, shortness of breath and reduced oxygen saturation in the blood. The RKI has compiled findings on the time between the first symptoms and more severe courses up to admission to hospital in its COVID-19 profile.

Overview on the subject of coronavirus (imago / Rob Engelaar / Hollandse Hoogte)

What does it depend on whether someone has only mild symptoms or needs ventilation in the intensive care unit with severe pneumonia? There are no hard data on this yet, but reasonable assumptions. Accordingly, there are three factors for the course of a COVID-19 disease.

1. The virus - dose and variants

The virologist Christan Drosten said weeks ago that it probably makes a difference how much virus you get and whether it gets deep into the lungs or has to work slowly from the throat, so to speak. There are experiments with ferrets that show that close contact is crucial and that animals that have only gotten a few virus particles through the air can indeed carry the pathogen within them, but do not get sick. At the beginning of the epidemic in Germany, employees of the Webasto company in Bavaria fell ill who had worked with an infected person in a seminar room for a long time. Taxi drivers or hotel employees who only had brief contact with the person are not ill. The dose makes the poison - that also seems to be true with COVID-19. However, it is still unclear what the virulent dose is. The so-called Heinsberg study, for example, which aims to precisely track chains of infection, could provide information on this. Then you will see how close the contact has to be for a transmission to take place.

There are also differences between the viruses. They mutate, change. Not as fast as flu viruses, for example, but still measurable. Initial reports of various aggressive variants were probably exaggerated. But a study has just appeared that gave different variants of SARS-CoV-2 to human cell cultures in the laboratory. All of them were able to reproduce, but some were much faster and more effective than others. The more aggressive variants could also kill the cells faster. So it probably depends not only on how much virus you get, but also on the exact genetic version of SARS-CoV-2.

(imago images / ZUMA Press / Cdc / Cdc) What we know about the dangerousness of virus variants
According to a Chinese study, there is a rather harmless variant of the SARS-CoV-2 pathogen and a more dangerous one. Virologists criticize this conclusion. The data cannot explain the many deaths in Italy either.

2. Human genes - portals of entry and immune system

First of all, there is the entry portal of the virus: a protein called ACE 2, which sits on the surface of lung cells and, by the way, of many other cells and serves as a receptor for the virus. There are variants, but no one has yet found out whether they have an impact on COVID-19. The amount of ACE 2 is more important. In fact, old people produce far less ACE 2. That sounds like good news at first - fewer entry points for the virus, fewer infections. But ACE 2 has important tasks - including controlling the immune response. It can become a problem here if there is little ACE 2 available. Because for many people who die of COVID-19, in the end it is not the virus itself that is the problem, but a defensive reaction that gets out of hand and destroys their own tissue, especially in the lungs. One thesis that is currently being pursued therefore says: Old people develop less ACE 2 and therefore may not be infected so easily. But if the virus catches them, the consequences are worse.

Subscribe to our coronavirus newsletter!

The risk of death is particularly high for older people, but younger people also die from COVID-19. One explanation for this is a differently positioned immune system. There is a particularly large variety of defense genes, each of which can recognize different pathogens particularly well. This means that every individual immune system has strengths and weaknesses. The different immune genes can also recognize SARS-CoV-2 to different degrees - this is suggested by a study published in the "Journal for Virology". The authors even suggest giving preference to vaccinating people with a poorly immune system at this point. The data is still too thin for that.

Defense cells of the immune system (imago) Why the immune system can be dangerous
The immune system protects our organism from unknown pathogens. However, it can also be directed against our own body and damage the lungs of seriously ill COVID-19 patients - with life-threatening consequences.

The most important genetic factor is the Y chromosome: men are more ill than women. It looks like estrogen itself could be a protective factor because it has a beneficial effect on the immune response. New York is already investigating whether the female hormone can help men with COVID-19.

What stands out: COVID-19 is not only a disease of the lungs, the virus also causes problems in the blood vessels. Younger patients develop thrombosis, even strokes. An existing tendency to have problems with blood clotting could be an important factor here.

(imago / Westend61) Coronavirus may also attack nerves and the brain
The symptoms of COVID-19 are varied. There are now increasing indications that the new coronavirus is also attacking nerve cells. It's not just about smell and taste disorders.

3. Everyday life of people - previous illnesses and living conditions

Since the virus apparently also attacks the blood vessels, existing cardiovascular diseases seem to be particularly problematic. Diabetes can also dampen the immune system and is therefore a risk factor. The doctors here agree that there is hardly an increased risk if these basic diseases are well controlled. In Germany there is broad access to diabetes therapies, antihypertensive drugs and so on. That could be one of the reasons for the comparatively rare really difficult courses.

(Pexels / rawpixel) "A well-adjusted blood pressure helps the body against the virus"
Antihypertensive drugs may increase your risk of developing COVID-19. However, doctors advise against stopping medication: uncontrolled blood pressure parallel to a viral infection is more risky.

In the USA, African Americans are particularly badly affected, as they are particularly prone to cardiovascular problems, obesity and diabetes. The poverty factor also plays a role here. Social factors are important for the course of many diseases, this also applies to COVID-19. Smokers also cope with COVID-19 more poorly, probably because their lungs are pre-damaged. Preliminary data also suggest that air pollution plays a role. The first hotspots of the coronavirus spread, Wuhan in China and northern Italy, are both industrial locations.

(dpa / Paul Zinken) How poverty and health are related
According to the RKI, people with little education, low income and occupational status are at two to three times greater risk of developing diabetes or cancer, of having a heart attack or stroke.