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A thief in the night: SARS-CoV-2 and immunity theft

When we catch COVID-19, is SARS-CoV-2 creeping around our bodies like a thief in the night, stealing our immunity? Is there such a thing as “immunity theft”?

As far as I could find, the term “Immunity theft” was coined by T. Ryan Gregory, a Canadian professor of evolutionary biology, as one of many possible explanations for the current unseasonably high (and severe) cases of RSV. In his words, “The concept of "immunity theft" is simply that COVID infection (let alone repeated infections) may rob people of a working immune system, which puts them at risk for other infections.”

In fact, “immunity theft” might just be a new name for an old concept, albeit with a much less catchy name. If you query “immunity theft” on Pubmed, a search engine for scientific papers, you won’t find any matches. But if you search for “compensatory anti-inflammatory response syndrome”, or “CARS” for short, you might find the information that you’re looking for.

CARS is a phenomenon that is already relatively well-established within the jargon-heavy field of immunology. It is a sort of sister term to “SIRS” (immunologists do love their acronyms), or “systemic inflammatory response syndrome”. While CARS is a calming-down of the immune system, SIRS is the immune system in superdrive.

Put simply, SIRS occurs when your immune system gets excessively hyper-active in an over-enthusiastic effort to get rid of some microbial threat. This hyperactivation of the immune system is classically seen during sepsis, or “blood poisoning”, when your immune system notices some microbe floating around in your arteries or veins and goes on high alert. However, just like if an army destroys an entire city in trying to root out an enemy, when an immune system is on excessively high alert, it can cause more damage than good to your body. And this is why CARS exists.

CARS is your body’s response to SIRS: a sort of under-active immune state that comes into play in order to try to restore some balance to your immune system. It’s sort of like the UN peacekeeping forces that might enter a warzone in order to get things under control again and restore some semblance of normality.

But what does this all have to do with COVID-19? Well, even in the early days of the pandemic, some researchers noticed similarities between what happens during cases of sepsis and what happens during severe cases of COVID-19. It is true for both sepsis and COVID-19 that our immune systems can actually do even more damage to our bodies than the microbe they are trying to fight off – causing serious symptoms like blood coagulation, tissue damage, or even multi-organ failure at its worst. When this happens, our immune system is trying to do well by us by attacking whatever microbial invader was detected. It’s just rather overzealous, and throws out the baby (us) with the bathwater (the microbe).


The similar symptoms of severe COVID and sepsis has now led some academics and health organisations to suggest that severe COVID-19 might actually be a kind of viral sepsis. And, as we learn more about “long COVID”, this suggestion only becomes more convincing, because some of the symptoms of long COVID are pretty similar to the symptoms experienced by patients recovering from sepsis.

So, it follows that CARS, with its hallmark of relatively low immune system function that helps with the recovery from the over-active immune system state in SIRS, might also be experienced by people recovering from severe cases of COVID-19. Or, in other words, “immunity theft” may just be the COVID-19 version of CARS.

Now, I need to stress again that this is just a hypothesis: there is no incontrovertible proof yet that immunity theft/post-COVID CARS exists, just as is the case with the hypothesis of “immunity debt”, as we covered last week.


But let's imagine for a moment that it is true. Let’s assume that people can experience CARS after a severe case of COVID-19. What should the public health response be? Well, it certainly would not make sense to make sure that more people are infected with SARS-CoV-2 – that would put those people at risk of being more susceptible to any given microbe, alongside any other potential negative side effects resulting from having first a hyper-active followed by a hypo-active immune system as may occur during sever COVID-19 cases.

Rather, I would say that the best approach would be to prevent the spread of SARS-CoV-2 and reduce the chance of people catching COVID-19 and experiencing any of the side effects that go with it. And yes, you guessed it, the safest and most effective strategy for preventing the spread of contagious diseases is to vaccinate against those diseases when possible.


So, just as with immunity debt, immunity theft may well be one of the many possible underlying explanations for the high amount of RSV cases this season. We just don’t know yet, and I would not dare to assign either immunity debt nor immunity theft with the full responsibility for the uptick in RSV infections. As I covered in last week’s blog, there are other possible contributing factors, and it's essential to be sceptical and unemotional when considering alternative explanations as a scientist. But regardless of what the cause of the uptick in RSV cases may be, I am sure about one thing. Whenever possible, I continue to prefer giving my immune system a controlled, careful basic training via vaccination, rather than sending it straight into an all-out war zone in the manner of a natural infection.



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