Home » Why have some people never taken covid? – Zania Stamataki

Why have some people never taken covid? – Zania Stamataki

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Why have some people never taken covid?  – Zania Stamataki

01 April 2022 15:51

I am one of the lucky few who has not yet tested positive for covid-19. This despite the fact that for my research I work in contact with the live and reproductive sars-cov-2, give lessons in presence at the university and have children who go to school.

I have healthy, fully vaccinated friends my own age who weren’t that lucky. And some have gotten sick more than once from covid-19 in the past two years. What does all of this reveal about my immune system?

Memory cells
First, we need to consider a number of scenarios. There is a very small chance that I never came in contact with the virus. But given the duration of the pandemic and the number of highly communicable variants, it is unlikely. Then there is the possibility that I came in contact with sars-cov-2, but that it was cleared from my body quickly before it developed and caused the disease (abortive infection). At the start of the pandemic, and before I was vaccinated, I may have caught the virus, but I may have been among the few people who showed no symptoms and therefore didn’t get tested.

Some people can eliminate the virus quickly because they have pre-existing antibodies and cells with an immune memory that recognize the virus. These could be cross-reactive T cells previously generated to fight coronaviruses responsible for the common cold. There is evidence of a higher prevalence of endemic coronavirus (non-covid-19) infections in young people and a reduced presence of cross-reactive T cells in older people.

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If I had previously contracted one variant, but reacted well, I am not convinced that I can be immune to the next one

Since the vaccines are available, I have received my first and second doses, as well as a booster injection. Vaccines work by exposing our immune systems to the virus’s spike protein, and setting in motion an early arsenal of specific antibodies and T cells. These leave behind memory cells, which can last for years and take action to prevent reinfection.

While covid-19 vaccines still protect against more severe forms of the disease, every time a new variant emerges, we scientists frantically try to figure out, from real-life data, whether it can evade vaccine protection. We cannot predict when the effectiveness of the vaccine will decline, because we do not observe the gradual evolution of the virus, in which emerging lineages add new mutations to their predecessors; the omicron variant, prevalent today, bears little resemblance to the delta, which spread widely last year. Natural infection offers no long-term protection, and the more potent vaccine-induced immunity needs a booster to protect against variants.

Consequently, if I had previously contracted one variant, but reacted well, I am not convinced that I can be immune to the next one. In fact, people report different symptoms after several cycles of infection: during subsequent infections, some feel better and others worse.

Polymorphisms of reactions
There is also the possibility that different immune systems respond differently to the virus. In order for sars-cov-2 to infect, the spike protein on the surface of the virus needs to attach to specific proteins on target cells, such as the Ace2 protein. Is it possible that people resistant to infection have different levels of Ace2 than other people? The lower presence of Ace2 in the lungs of children, compared to adults, could partially explain why children often show a milder infection.

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It is also possible that some of us have rare types of Ace2 that the coronavirus spike protein cannot attach to. The differences in how people react to the protein are known as polymorphisms, and their discovery is invaluable. People who have a rare genetic polymorphism with respect to the CCR5 protein have been found to be immune to HIV infection. In support of this theory, recent genetic analyzes have revealed that rare types of Ace2 can influence the possibility of covid-19 infection.

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Studies on health care workers who remained consistently negative for covid-19 also showed the presence of pre-existing T cells that recognize peptides – the chain of molecules that make up a protein – from parts of the virus that are less variable than the spike protein (which, under the pressure of our immune response changes frequently to evade our antibodies). These studies suggest that it would be wise not to rely on vaccines that target the spike protein if we are to build immunity to new variants, and we should consider incorporating more parts of the virus that do not change over time (evolutionarily conserved proteins) into the design of the our vaccines.

We are still struggling to understand what can cause covid-19 resistance, and we can’t be sure why someone like me hasn’t tested positive yet. But what I do know is that since new variants are likely to emerge, there is still no guarantee that I will not be infected. While we’ve been lucky so far, it’s best not to take any chances.

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(Translation by Federico Ferrone)

This article appeared in the British newspaper The Guardian.

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