The coronavirus SARS-CoV-2 that causes COVID-19 has infected nearly 2.5 million people worldwide and claimed 170,000 lives.
But some people do not even have symptoms. Recent studies have shown that up to 80% or more of the infected people are "silent carriers" with no or very mild symptoms. It seems that children and young healthy people are more likely to be asymptomatic.
However, to calculate the true proportion from asymptomatic to severe disease, it is necessary to extend the detection to the entire population, which is not feasible.
We do not know why some carriers of coronavirus have no symptoms, while others have life-threatening diseases. But the following is what we currently know.
Like all viruses, SARS-CoV-2 needs to enter the human cells to reproduce and survive. To do this, a particle on the virus's shell attaches to a protein receptor called ACE2, just like a lock and key. ACE2 receptors are usually found in the lungs, kidneys, heart and intestines.
Once a person is infected with this virus, it may take up to 14 days for symptoms to appear (if they do show symptoms). This is called the incubation period.
The path from the point of infection can be very different. The body's immune system is the key to this.
During the incubation period, there is a strong immune response that prevents infection, reduces the actual amount of virus in the body, and prevents the virus from entering the lungs.
The first is the innate system, which includes physical barriers such as skin and mucous membranes (the inner layers of the throat and nose), various proteins and molecules found in tissues, and some white blood cells that attack invading organisms. This immune response is universal, non-specific, and will occur soon.
Children's immune system is immature, but there is a hypothesis that explains why they don't seem to get COVID-19 easily, that is, their innate immune response to coronavirus is stronger than adults. This may result in a reduction in viral load (the number of virus particles living in the body) because they can clear the virus faster.
The second line of defense is the adaptive immune response. This takes longer to start, but once established, it can eradicate infections more effectively when it encounters the same pathogen again.
It is believed that very specific genetic variations in some people may be one of the reasons for their illness. By generating an early adaptive immune response, the human body appears to be able to recognize the virus during the incubation period and repel it.
A person also needs to be healthy to have an appropriate immune response to infection.
If the SARS-CoV-2 virus survives after entering the body (nose, eyes, throat), it may enter the lungs through the respiratory tract.
In the lungs, it attaches to the ACE2 receptor and continues to replicate itself, triggering a further immune response and clearing infected cells. The amount of virus that penetrates the lungs may be another important factor in determining the extent of your illness.
As the battle between the virus and the immune response progresses, the infected airway endocardium produces a large amount of fluid, filling the alveoli, leaving less space to transfer oxygen to the blood and remove carbon dioxide. What follows is the appearance of symptoms of pneumonia, such as fever, cough, phlegm, and shortness of breath.
For some people, the immune response is excessive or prolonged, leading to a so-called "cytokine storm." Cytokines are a group of proteins that send signals to cells in the immune system to help direct reactions.
Cytokine storm is a catastrophic overreaction, it can cause a lot of inflammation and organ damage, even fatal.
In patients with COVID-19 and Middle East Respiratory Syndrome Coronavirus infection, when fluid accumulates in the lungs, it can cause acute respiratory distress syndrome (ARDS). This is the most common cause of death from SARS-CoV-2.
Older people and people with chronic lung disease are more likely to develop ARDS and die. It is currently believed that this is due to the fact that these people have fewer ACE2 receptors in their lungs.
This seems to be counterintuitive, because viruses attach to these receptors. However, the ACE2 receptor plays an important role in regulating the immune response, especially controlling the degree of inflammation.
Therefore, the reduction of ACE2 receptor levels in the elderly may actually make them more vulnerable to cytokine storms and serious lung diseases.
In contrast, children have more ACE2 receptors in their lungs, which may explain why they do not get sick.
In some cases, drugs that suppress the immune system have successfully treated this excessive immune response in COVID-19 patients.
Some studies have shown that patients with COVID-19 tend to have higher viral loads before and shortly after symptoms appear.
This shows that they can spread the virus 48 hours before the disease or even earlier, and when they are in the pre-symptomatic period. However, there is no good evidence that people without symptoms can spread the virus.
Researchers and clinicians are working around the clock to understand the complex relationship between the human immune system and SARS-CoV-2, but work in this area is still ongoing.
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