Coronavirus and Covid-19: why are some people not infected and others more than once?

The Human Challenge study from Great Britain

A year ago, Imperial College London’s coronavirus project sparked the debate. For the so-called human health study, 36 young people between the ages of 18 and 30, who were neither vaccinated nor cured, were intentionally exposed to the corona virus by dripping a small amount of an original variant of the corona virus into their noses.

The subjects were then isolated for two weeks, medically monitored, and repeatedly tested for the virus. This was followed by another 12 months of observation.

The surprising thing about this experiment: only half of the people in the test actually contracted the corona virus and developed the corresponding symptoms. On average, these occurred approximately two days after infection, and thus significantly earlier than observed in the first waves of infection. However, this could be because subjects paid more attention to whether they were infected.

T cells and immunity to related viruses

The question remains as to why only half of the test subjects were infected, while the corona virus apparently was unable to multiply in the other half of the test subjects.

There are several theories about this. The genetic characteristics of some individuals may play a role. It would also be conceivable that an existing immunity against related pathogens – for example the common corona viruses of the cold – they protected them from infection or a symptomatic course.

So called T cells a role. These are special white blood cells which, partly in interaction with antibodies, take on tasks of the immune system. They are able to make infected cells in the body harmless and prevent the further spread of a virus.

T lymphocytes create an immunological memory. Even years after an infection or vaccination, they can recognize and fight pathogens. But it doesn’t seem to work for everyone.

Likewise, existing immunity to some influenza pathogens could offer some protection against the corona virus.

Different types of fabric

Some people owe their immunity to their HLA tissue type. It also plays a role in transplant medicine and is critical in determining whether an organ donor and an organ recipient are compatible.

A certain type of tissue may therefore be able to immediately render coronavirus-infected cells harmless or simply be particularly susceptible to it.

When viruses enter the body and infect a cell, the cell starts producing interferons. This puts all neighboring cells in an antiviral state. This works especially well for children. That is why they play a rather subordinate role in the pandemic.

Further explanations

But these aren’t the only explanations for why some people seem to contract the corona virus less quickly. Due to the particular situation of the study, the small number of participants, and the young age of the test subjects, the Human Challenge study does not cover all factors that could affect the likelihood of a corona infection.

Virus amount and threshold

For the British Human Challenge study, all test subjects were exposed to the same amount of the virus. This is not the case in everyday life. The amount of viral particles you ingest depends on the duration and type of encounter with an infected person.

To get infected, you have to come into contact with a certain dose of a pathogen. This means: A certain threshold must be exceeded. The higher the viral load of an infected person, the greater the likelihood of exceeding that threshold and of being infected if encountered. If people have an immune deficiency, their natural threshold is lower.

Whether you get infected with the coronavirus also depends on the number of viruses you come into contact with, whether you cross the individual threshold and how your immune system works.

The role of the time of day

The effectiveness of the body’s defenses depends, among other things, on the time of day when you come into contact with the virus. At night, when we sleep, our immune system regenerates. Therefore, the likelihood of a viral infection in the morning and mid-morning is lower than the next.

Age factor: the older, the more vulnerable

As we age, the performance of our immune system declines. From the age of around 65, the so-called immunosenescence comes into play, with which the risk of infection increases. This gives the virus the ability to spread faster.

Women less affected than men

Women have stronger immune systems than men. Therefore, they are also less susceptible to Covid-19 infection and are less likely to die from it. The death rate from Covid-19 is disproportionately high among men.

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