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Claims of an Immunity Debt in Children Owe Us Evidence

As hospitals fill up with sick children, we are told pandemic measures understimulated their immune systems. But where is the evidence?

At the beginning of the fifth season of the television show Buffy the Vampire Slayer, we the audience are introduced to a character, Dawn, we have never seen before. Intriguingly, every character around her acts as if she has always existed. We know that something is amiss, but her friends and relatives don’t.

I feel the same way about the concept of an “immunity debt.” It sprang out of nowhere, but so many people act as if this is an old chestnut in the field of immunology.

Basically, as paediatric hospitals overflow with sick children, the media is serving us a possible explanation: all of that masking and virtual schooling and physical distancing created an immunity debt in children, and it’s now time to pay the piper. Even PediaSure® is getting in on the action. One of their brochures claims that “kids staying indoors for long prevents exposure to common bacteria causing immunity debt.” Their solution? Two scoops a day of PediaSure® to help build your kids’ immunity! How convenient.

Is there any truth to the idea of an immunity debt? Depending on how you interpret this rather ambiguous phrase, the answer can be yes, but we must first dispel the Dawn-like myth that knowledge of this putative vulnerability has always been with us.

An old idea from 2021

You can search the Internet for “immunity debt” every which way: you are not going to find a reference to it pre-COVID. A Google search I conducted had a few pre-pandemic hits, but they were to older news articles that are now accompanied by updated suggested readings of recent media reports of this alleged immunity debt. A query into PubMed, a search engine for papers published in the life sciences, resulted in 11 hits, all from the past two years. A Twitter user also did a thorough search of digital spaces, coming up with very little. It turns out that that infamous phrase gained life in a position paper from a French team published in the journal Infectious Diseases Now and first published online on May 12, 2021. I will refer to it as the 2021 Cohen paper, as its first author is Professor Robert Cohen.

This paper is an interesting example of the motte-and-bailey fallacy. You can imagine a medieval castle on a mound (the motte) and the land around it (the bailey). It is harder to defend the land and much easier to retreat to the castle and protect that instead. A motte-and-bailey fallacy occurs when a strong argument is made and, in response to or in anticipation of pushback, the person making the argument retreats to a weaker version of their argument and defends that instead.

The 2021 Cohen paper firmly asserts that immunity debt is real and it is caused by the lack of immune stimulation due to pandemic-related measures. There is no equivocation on the part of the authors… until the final page of their paper. All of a sudden, certainty gives way to a weaker position: that those pandemic measures “may” have led to less training of children’s immune systems. In the following paragraph, the word “hypothesize” is used for the first time in this context.

The evidence presented for this immunity debt due to understimulation—advanced first as a certainty, then as a hypothesis—is lacking. But this paper, boldly asserting the existence of an immunity debt in children, opened the floodgates, and soon it was being quoted in other papers and in media reports, and now we are led to believe that our immune system is just like a muscle: stop working it out and it will atrophy. This is the challenge of science communication. Simple analogies stick in people’s minds, even when they are wrong. Our immune system is not like a muscle. It does not require constant poking and prodding from a germ to avoid lethargy.

And even if it did, children during the pandemic were not kept in sterile bubbles. They were in contact with microorganisms from the food that they ate, the soil that they played with, and the adults in their lives. The idea that Zoom schooling created an aseptic environment is silly on its surface.

There is one way, though, in which a lack of contact with some disease-causing microorganisms may have somewhat reduced a person’s overall immune response compared to what it would have been otherwise, and this element is cited in the 2021 Cohen paper. The first line of defense of our immune system—before antibodies come to the rescue—is our innate immunity, long thought to be a mindless infantry that never learns. But there is accumulating evidence that this arm of our immune system can learn and that exposure to one vaccine, for example, can enhance our innate immunity’s ability at fighting off a completely different infection in the future. This is known as trained immunity.

It would be tempting to take this emerging finding and conclude that it wholly explains our current situation: COVID-19 measures decreased the number of germs that children were exposed to, and these kids’ innate immunity is now less prepared to deal with respiratory viruses today.

The ambiguity of the phrase “immunity debt,” however, means that there is another, much more relevant interpretation of it.

A lot of catching up to do

One of the germs putting children in hospitals these days is a virus that many people may not be familiar with: respiratory syncytial virus or RSV. It is the most common cause of acute respiratory infection in infants, and almost all children (barring a pandemic) have been infected with RSV by the time they are two years of age. There is no vaccine for RSV currently available, although several seem to be on the cusp of being approved.

Part of the reason why so many children are ending up hospitalized with RSV now is that for the past few years, their exposure to the virus was minimal due to public health measures. With life returning to normal, children who would have been infected by RSV for the first time a year or two ago are instead encountering the virus now, and it is the first infection that is typically the worst and thus more likely to land them in the hospital. As Dr. Fatima Kakkar, an infectious disease paediatrician at Sainte-Justine Hospital, explained on CBC’s Front Burner podcast, instead of all of these children getting the virus in a staggered manner over the past two years, with hospitals able to cope with it, “everybody is getting it at the same time and they’re getting it for the first time. That’s why it’s so severe.”

And it’s not just RSV that is putting kids in the hospital but respiratory enteroviruses, influenza, and parainfluenza as well. If by “immunity debt” we now mean that these are viruses that many children were not exposed to before and they are now all being infected with them at the same time, then yes, there is a type of immunity debt, I suppose. A big “pause” button was pressed for two years and there is now a lot of catching up to do.

An added twist is that mothers were also less exposed to viruses like RSV during the pandemic. They can pass down protective antibodies against the virus through the placenta to their child. While these antibodies acquired in utero do not prevent infants from being infected by RSV, they can protect them from the more severe complications that would require hospitalization.

Hospitals have also been under tremendous stress these past few years, with less money and more healthcare workers calling in sick due to COVID. Add to that a shortage of over-the-counter pain and fever medication for children, and we have a recipe for a paediatric hospital emergency without having to invoke the dubious notion of an immunity debt.

And while this alleged immunity debt can easily be weaponized to dispute the need for public health measures, the authors who coined the phrase in 2021 were using it to argue for vaccination. The pandemic forced many parents to delay getting their young children vaccinated. The paper’s authors conclude this should “lead to the implementation of reinforced catch-up vaccination programs,” the word “reinforced” here alluding to France’s more limited vaccination schedule which lacks immunizations against rotavirus, varicella, and certain types of meningococcal diseases. If children did not receive their vaccines because of the pandemic, it puts them at risk for vaccine-preventable diseases, which can increase the number of children in need of hospitalization.

The idea that keeping children at home in the middle of a pandemic did irreversible damage to their immune system is a seductive one for the people who keep arguing that COVID is just a bad flu and that it rolls off kids like water off a duck’s back. “They need to be exposed to germs,” these people will tell us, often invoking an oversimplified version of the hygiene hypothesis. But as the famous webcomic xkcd pithily put it:

CHARACTER 1: “See, it’s good to get infected, because it gives you immunity.”

CHARACTER 2: “Why would I want immunity?”

CHARACTER 1: “To protect you from getting inf…. Wait.”

We have been vaccinating people against measles, diphtheria, polio, rotavirus, influenza, hepatitis, and many more diseases, stopping their infectious agents from naively interacting with our immune system, and yet we do not seem to have incurred an immunity debt as a result of these vaccines. On the contrary, we live longer and healthier.

The concept of an immunity debt sprang out of nowhere in 2021. In March of this year, many of the same French researchers published a follow-up paper and added another reference to back up their claim for the existence of this debt: the introductory chapter of a book called Primer to the immune response. It does highlight types of immunodeficiencies, such as those caused by a genetic defect and those acquired via a nutritional imbalance or a disease-causing agent, like HIV. But as far as I can tell, it makes no mention of the idea of an immunity debt.

I feel like we are being gaslit.

Take-home message:
- Some people believe that public health measures against COVID-19, including masking and staying home, have resulted in an understimulation of children’s immune systems called an “immunity debt,” and that children are getting more sick now because of it
- The phrase “immunity debt” comes from a French position paper published in 2021 and no evidence was cited to back up this claim
- It’s not that children’s immune systems are weaker, but rather that they are all being exposed to viruses from which they were shielded when public health measures were in place, and they are now falling ill at the same time


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