Many Covid-19 cases are asymptomatic – but how does that happen?

• Data shows that a large number of people infected with SARS-CoV-2 do not experience symptoms.
• However, these people are still able to pass the virus on. Hence, it is important to understand how this is done.
• According to a new study, the answer may lie in the virus’ ability to prevent its genome from being recognized.

People who contract Covid-19, the disease caused by the SARS-CoV-2 virus, suffer from mild to serious illness or even death – and then there are people who have no symptoms at all.

A recent study published in PLOS medicine, estimated that 20% of SARS-CoV-2 infections are asymptomatic (showing no symptoms) but still contagious, which begs the question, how does this happen?

According to new research from Aarhus University and Aarhus University Hospital in Denmark, the virus may prevent its genome (genetic material) from being recognized.

The results were published in EMBO reports.

To gain a deeper understanding of the phenomenon, the researchers examined specialized immune cells called alveolar macrophages (AMs) that are found in our lungs. They form an important defense against pathogens in the lungs.

After one in the American Journal of Respiratory and Critical Care Medicine, AMs clean the air spaces of infectious particles that have eluded the mechanical defense mechanisms of the airways, such as B. the nasal passages.

Because the lungs contain significant numbers of these immune cells, they are likely the first cell type to be encountered by an invading virus, the researchers said.

The team took a deeper look at interferons – a group of cytokines that are essential to fighting viruses. When our body detects a viral infection, our immune system initiates the production of these interferons.

Previous research has shown that AMs produce large amounts of interferons when infected with respiratory viruses such as influenza.

According to a press release from Aarhus University, new research also shows that interferon production in the infected epithelial cells can be inhibited by the new coronavirus, which typically infects the epithelial layer – the outermost cell layer of the lungs.

Although the epithelial layer is the virus’ target, the researchers suggest that it must be assumed that the first type of cell the virus encounters are the AMs. These cells therefore play an important role in how quickly an immune response to a SARS-CoV-2 infection can be triggered, they explain.

The process

With all of these in mind, the team set out to investigate how these cells respond to SARS-CoV-2.

For this purpose, AMs obtained from bronchoalveolar lavages (BAL) – also known as lung lavage or lung wash – from donors diagnosed with non-infectious lung diseases were exposed to SARS-CoV-2.

They then examined the activation of the immune system in these cells when the coronavirus appeared.

New coronavirus can hide its genome from detection

Alveolar macrophages have the potential to produce large amounts of interferons during a viral infection such as influenza. However, the researchers did not see any interferon production in the cells when the alveolar macrophages were exposed to the SARS-CoV-2 virus.

Therefore, this suggests that SARS-CoV-2 may prevent its genomic material from being recognized in the alveolar macrophages in some people. As a result, it does not induce interferon production, the authors say.

This explains why the immune system is not activated in the early stages of SARS-CoV-2 infection, ultimately allowing the virus to spread further in the community before symptoms appear in individuals, the researchers explained.

However, they added that more research is needed to understand how the virus cannot be recognized by the immune system.

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