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.
I've done a lot of google searches - in fact, it is not known, for what physiological or immunological reasons there are persons who "just won't get sick" from CoV-2, and the answer to your questions 2 and 3 should be "yes" as any better answer would be speculative (yes, ... they can fight COVID-1 ... yes, they have a good immune system to fight coronavirus).
Expanding your question to "silent transmittance" might lead to more insight. There is related question at SE Biology: "Explain symptomless transmission..."
My own personal opinion might be helpful in showing that the reaction of the immune system you refer to - "cytokine storm" - which causes symptoms can be distinguished from the loss of function of the cells that get killed by viral invasion. Basic knowledge is that infected cells are killed in self-defence by immune cells who act on "presenatation" of antigen by infected cells. Thus, there is more or less so called apoptotic dying of cells causes by the virus and/or the immune "killer" cells (same with vaccination, by the way, to a lesser extent). If there is dying of body cells who in case of Covid have a certain function in "regular life" there should be people who do not only differ in the power of their immune system - so my answer is more than just two yesses to the second part of your question - but who's reputation against CoV2 schuld lie in their better coping with a loss of function of maybe many target cells of the virus.
Those cells are ACE2-bearing cells, different from blood pressure regulation cells of the vein's epithelia (targeted by ACE-inhibitor medication)which carry the same membrane proteine. The ACE2-protein is a marker of lung epithelial cells, called pneumocytesII, that act as stem cells and in regular life do not only renew lung epithelial cells (pneumocytes type I) but also produce "slimy" surfactant. It is only my guess, and there is no authority available according to my searches that "dry cough" as a known symptom of COVID-2 is caused by loss of surfactant because of pneumocytes II cells being killed by the virus. So people might differ in their power to cope with a loss of those surfactant producing stem cells. Beware that there is no authority on that, according to negative results of my internet searches.