Why Omicron symptoms are less severe than other Covid variants

When Omicron was first identified in Botswana and South Africa, two central questions arose: whether this new variant was more contagious than previous ones and whether it caused more or less serious illnesses.

How quickly the variant has spread and how quickly the cases have increased, it is clear that it is more contagious. Somewhat more complex, however, is the question of whether omicron has less harmful effects than previous variants – whether it is less “virulent”.

First results from South Africa (pending review) suggested that patients with Omicron are less likely to be hospitalized than before. And research has shown that even if people are admitted, less possible Need oxygen, need mechanical ventilation, be admitted to the intensive care unit, or die.

But South Africa has a young population compared to most of the world. A relative lack of older, more vulnerable people to contract the virus may have masked the variant’s capabilities. It was initially unclear whether Omicron would also lead to minor illnesses in other countries.

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However, the information gathered over the last month has shown that the South Africa experience is not an anomaly. Data from most European countries, including the UK, support the belief that Omicron is generally less likely to cause serious illness than previous variants.

This is most evident in the recent evolution of the epidemic in the UK. analysis the British Health Security Agency assumes that Omicron is only about a third as likely to lead to hospitalization compared to the Delta variant.

Of course, if the number of daily omicron infections increases to a very high number, as it does, hospital admissions will increase too. In the final days of 2021, hospital admissions of COVID-positive people rose significantly, but still not over Levels seen in the previous waves. And most importantly, we have not yet seen the number of patients occupying beds with mechanical ventilation increasing.

Partial immune bypass

Several different factors could explain why this happens. First of all, there is immunity. It could be that previous immunity, which is much more common today than in previous waves, thanks to previous infections and vaccination programs in many parts of the world, is sufficient to lower the rate of serious illness.

In order to protect against an initial infection, which are localized in the nose and throat, antibodies against an important part of the external structure of the coronavirus – its spike protein – seem to play the greatest role. This, however subside over time. Also, many of the mutations found on Omicron affect the spike protein, as well as likely to affect the ability of the antibodies to bind to the virus, Decrease in effectiveness a vaccine or previous infection to prevent new infections.

But to protect against a more severe infection, which is usually localized in the lungs, there could be other immune responses play a more dominant role, such as Killer T cells. These are a type of white blood cell that kill both foreign substances and cells that have become infected with the virus.

And early research, which has yet to be verified by scientists, suggests that omicron very few mutations to the parts of the virus that T cells target. This implies that the immunity built up in the past, which is potentially more effective against serious diseases, should still be largely intact.

This could be a strong explanation for why Omicron’s virulence appears lower. In the UK, for example, it is estimated that almost 95% of adults have some form of immunity to the virus through vaccination or infection.

Mutate to mildness

But could Omicron in itself be less virulent than previous variants, even in those without prior immunity? This is more difficult to judge, although there are some indications that it is.

First, Omicron shares several mutations near part of its structure called the furin cleavage site, which is believed to play a role in virulence. In laboratory experiments in which the furin cleavage site of the virus was deleted, the virus showed reduced growth in human airway cells and causes less serious illness in laboratory animals. It is plausible that the mutations around the furin cleavage site could explain some of the reduced virulence of Omicron.

Several groups also have that Ability of omicron (or a similar pseudovirus ) to grow in tissue cells. These studies – also awaiting review – report that the virus grows less well in lung tissue but better in upper airway tissue than previous variants.

Increased growth in the upper respiratory tract could lead to more virus being expelled through the nose and mouth, which could partly explain why Omicron is more contagious. And since an infection of the lungs and not the upper respiratory tract is associated with a more severe illness, the variant’s reduced ability to grow in the lungs could also be an important factor in explaining the reduced virulence.

Overall, the evidence that Omicron causes less serious illness than previous variants, regardless of the mechanism, adds up. But that doesn’t mean we should view it as less of a threat. Ultimately, Omicron’s public health impact depends on how many people become infected. Because it is much more contagious, it can still happen that many people become seriously ill, even if the proportion of infected people who become seriously ill is lower. The need to contain the spread of the virus remains, at least for now.

Paul Hunter, Professor of Medicine, University of East Anglia

This article was republished by The conversation under a Creative Commons license. read this original article.

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