Why are most people who contract COVID-19 asymptomatic?
Does it mean that they can fight COVID-19 and recover fast?
Does it mean they have a good immune system to fight coronavirus?
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At this point in time no one can say for sure why many people appear to have asymptomatic infection, and figures range from 20-80% though part of the problem appears to be from false positives in antibody tests. Nevertheless there have been large numbers picked up on rt-PCR who carry the virus but at the time do not show symptoms. Some, if not most, go on to develop symptoms so they are the pre-symptomatic.
We know there's an age distribution so that those under the age of 5, and those over the age of 60 are more likely to have severe disease, and we know women have less severe disease than men.
We also know that also the virus attacks cells via the ACE2 receptor, as one of its attack points, and these are more numerous in the young, and in women. So, one could posit that for any given dose or innoculm of virus, then the higher the dose relative to the number of ACE2 receptors, the worse the disease might be. So, this would explain why it's worse in very young children as they have a smaller body size and fewer cells relative to the inoculum, then as you get older you have the peak number, and then as you age the numbers drop off again so the dose becomes relatively higher.
Then we now know the virus is mutating rapidly and this may explain why symptoms and disease severity may differ rapidly. And even in the same person you may have a number of different mutations present.
And then there are the odd observations that some children have had asymptomatic viral pneumonia picked up on CT scan. So, they're developing lower respiratory tract disease without fever. Fever, muscle aches and pains are often on account of cytokines released during the innate immune response which appears to be highly variable in this disease. Many people who have died go on to develop a highly active innate immune response called a cytokine storm which causes death.
In all the immune response appears to be highly variable, and there are host factors including co-morbities, that influence the disease response as well as mutations in the virus itself.
The reason why many people who contract COVID-19 are asymptomatic is simple. It is because your immune system can kill the pathogen before you show symptoms. In other words, your body overwhelms the virus before it takes over larger areas of your body.
The answer to your second and third question is yes and no. Most people, that recover quickly without showing symptoms, signifies that they had a good immune system to fight it.
This is the general response to any pathogen that enters your body and is not distinct to COVID-19. Your immune system immediately starts fighting the pathogen off. And if your body is able to find the antibody fast enough, the pathogen will be immobilized and will be restrained from infecting more cells.
My source is these immune system videos:
Recent studies have reported that about 80% of people infected with SARS-CoV2 are asymptomatic, it means they are "silent carriers". These patients show no or very mild symptoms. As it is known that viruses need to get into living cells to divide and survive. Same applies with SARS-CoV2, this virus attaches its spike protein present on the outer shell with the human cell's protein receptor, called Angiotensin converting enzyme 2 (ACE2). These ACE2 receptors are normally found in the lungs, kidneys, heart and gut. After getting infected with this virus, it take incubation period of about 14 days for symptoms to arise. Having healthy immune system during this incubation period can reduce the viral load and thus prevent it from entering the lungs.
Our immune system provides us two lines of defense. First one is innate component including physical barriers such as skin and mucosal membranes, variety of proteins and some of the white blood cells for attacking foreign material. This immune response is non-specific and works fast. The second one is adaptive component. This works slowly but is specific for particular infection and holds memory of infection.
Specific genetic variations in humans might also play role in determining the intensity of sickness. By producing fast adaptive immune response, the body may recognize virus early and work against it. General health also determines the immune response.
If SARS-CoV-2 virus still survives during this entry to the body, it makes entry into lungs via mucosal respiratory tract. There it binds to ACE2 receptors and replicates further, which triggers immune response. The amount of virus which one gets into lungs also determines severity of sickness.
In many patients, the immune response is intense which results in "cytokine storm". Cytokines are proteins that act as signals to generate immune response. This can lead to excessive inflammation which can cause organ damage and can be fatal.
According to my google searching there is no authority on physiological or immunological reasons for asymptomatic infection with CoV-19, so general principles should apply.
Individuals may differ in their immune system, but also may differ in their coping with a loss of normal cell function.
Refering to the immune system:
People differ genetically in their MHC, major histocompatibility complex. T-cells that kill infected cells to prevent further - symptomatic - spreading are being activated by a signal that is a combination of the specific viral antigen and the individual's specific MHC. That might explain why all - other factors equal - people will have different affinity of their killer t-cells. These t-cells also (as T4-Helper-cells) activate antibody production by B cells which should lead to indivually different patterns of antibody production.
Compare e.g. https://link.springer.com/chapter/10.1007/978-0-387-39241-7_10 "By predicting the immunodominant peptides from various common viruses we found that different MHC alleles are expected to provide quite different levels of protection."
Refering to the functions of normal body cells killed by virus: CoV-19 specifically attacks ACE2-cells which not only produce surfactant that enhances breathing but, functioning as stem cells, renew normal lung pneumocytes I. It seems evident that with age and existing diseases people differ in their coping with a loss of a certain amount of this functionality. What's more, infection might be stopped before CoV-19 enters the blood systems and affects different functionality of ACE2-cells that are part not of the lung but of the blood vessel epithelia. There again, people might differ in preexisting diseases.
Some hint from a beginner's standpoint: any reaction of the immune system, be it innate or adapative, litterally needs at least some infection of cell, thus replication of the virus, thus infectivity, of the virus, to start up the defence mechanisms - as the infection of some cell triggers the defence. Thus, symptomless transmission, surprisingly, appears as the rule, not the exception! You'd need some level of neutralizing antibodies present in your blood,in your lung epithelia, e.g. some recent preinfection leading to antibodies present, to possibly fend off the infection of cells and the "incubative", symptomless infectivity.