Earth in space surrounded by coronavirus particles

December 2, 2022, by Brigitte Nerlich

Immunity debt: Creating and contesting metaphors

This week I am writing a post about my probably last Covid metaphor: immunity debt. What do people mean by that, I wondered? While trying to find out, I became aware of how slippery a concept this is; so I apologise in advance for misunderstandings.

In 2021 a French group of researchers published a paper entitled “Pediatric Infectious Disease Group (GPIP) position paper on the immune debt of the COVID-19 pandemic in childhood, how can we fill the immunity gap?” This appeared as an e-publication in May and as a proper publication in August in the journal Infectious Diseases Now. They define immunity debt as “the lack of immune stimulation due to the reduced circulation of microbial [species] and to the related reduced vaccine uptake.” I think this mostly applies to a reduction of immunity at the level of a population, but there are also some speculations about changes in immunity at the level of individuals.

The phrase ‘immunity debt’ jumped from that scientific publication into the media and went viral, especially on Twitter. On 7 July 2021 The Guardian mentioned a worrying resurgence of the Respiratory syncytial virus (RSV) in children in New South Wales in Australia: ”Frightening story coming out of New Zealand: hospitals are experiencing the payoff of ‘immunity debt’ created by Covid-19 lockdowns, with wards flooded by babies with a potentially-deadly respiratory virus, doctors have warned.” Many more stories followed focusing on a surge of respiratory viruses in many countries around the world, affecting children in particular and using immunity debt to ‘explain’ this phenomenon.

The metaphor of immunity debt

The metaphor ‘immunity debt’ is part of a well-established compounding strategy around immunity, including scientific and popular compounds, such as ‘immunity system’, ‘immunity gap’, ‘immunity wall’, ‘immunity boost’ and many more, which all need exploring. But what about immunity debt? What does debt mean here? And how is the metaphor used and contested?

Before we dissect the metaphor a bit further, it is necessary to stress that it is perfectly well-established that, as Ryan Gregory has pointed out: “If a lot of people avoid infection in one season, then there will be less immunity in the population and larger numbers of susceptible hosts in the following season.” So, at the population level the concept works to some extent, although there is no ‘debt’, and we still have no real explanation for the apparent pattern of infections with respiratory viruses at the moment.

In immunity debt, ‘debt’ seems to mean, at least in some circumstances, that if you, as an individual, have not been exposed to a virus for a while and have stayed healthy, that health is a debt you have to repay or ‘pay off’ at some point by getting ill – ‘Immunity is money’. In the specific case of Covid, it seems to mean that people who avoided falling ill with Covid by staying socially distanced and masking, also avoided getting other viruses. Children in particular didn’t succumb to all the coughs and colds they usually get in the winter. Now children are falling ill with RSV for example and they are, apparently, repaying that immunity debt acquired during lockdowns. As the Wall Street Journal said on June 28: It’s like “borrowing from the future by creating greater room for viruses to run rampant later”. This also implies that lockdowns, masking and so on are somehow ‘responsible’ for children getting ill now. This claim is rather contested.

As Professor Peter Openshaw, a scientist specialising in lung immunology, is quoted as saying in an article by Anjana Ahuja for the Financial Times: “to frame this as an immunity debt, Openshaw warns, mistakenly suggests ‘that immunity is something we need to invest in, and that by protecting ourselves from infection we are building up a deficit that has ultimately to be repaid. This would not be a good message for public health: we would still have open sewers and be drinking from water contaminated with cholera if this idea were followed to its logical conclusion.’” He highlights some of the nefarious financial entailments of the metaphor, but he doesn’t use an alternative metaphor to counter the debt metaphor. Others did, as we shall explore now.

Contesting the immunity debt metaphor: Muscles, photographs and inflation

As we have seen, the paper by the French team of paediatricians seems to have been the trigger for a viral spread of the ‘immunity debt’ phrase. As Jonathan Jarry at McGill, Canada, said on 11 November: “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.”

Reading this made me prick up my ears, ‘muscle’…. I hadn’t seen that word in the original paper – where did this metaphor – ‘Immunity is a muscle’ – come from? Interestingly, it seems to have come from a critique of the paper, as far as I can ferret out. Colin Furness, an epidemiologist working at the University of Toronto, had  posted a thread on Twitter on 6 November about the metaphor and wrote: “A word on the idiocy known as ‘immunity debt’. How did such a nonsensical idea take hold? Simple. The obvious mental model for our immune system is a muscle: use it or lose it. Muscles atrophy when we don’t use them. The same mental model works for our memories & skills.”

But, as he also explains in the Global News, Canada on 12 November, that model/metaphor is wrong and he proposes an alternative metaphor. Furness points out that people imagine “that the immune system is like a muscle where, if you’re not using it, then you lose it, it atrophies”. The newspaper goes on to say that, instead, “Furness likens the immune system to a collection of photographs. When people take photos and put them away in an album, the photos don’t fade over time just because they aren’t being looked at regularly. With age, the immune system does start to become less effective at preventing illness, in the same way an elderly person might find it harder to see their photos as their eyesight weakens.” (For a more detailed unfolding of that metaphor, see here)

On 23 November Anjana Ahuja published an article critiquing the concept of individual immunity debt in the Financial Times claiming that “there is no evidence that an individual is worse off for having avoided earlier infection”. In her article, the muscle metaphor reappears in the mouth of another immunologist: “’Immunity debt as an individual concept is not recognised in immunology,’ Dunn-Walters says. ‘The immune system is not viewed as a muscle that has to be used all the time to be kept in shape and, if anything, the opposite is the case.’”

But what about money? The immunity debt metaphor is about money. Did nobody home in on that? Of course, people did. In the Globe and Mail two medical experts point out that children are not currency:

“Critics of public-health measures have seized on that language to argue that the current surge shows the folly of not letting COVID-19 and other viral diseases spread ‘naturally,’ and argue that the purported solution is to repay this ‘debt’ quickly by allowing infections to spread now. This revisionist history is nonsensical, and the proposed solution – turning sick children into a form of communal currency – is perverse.”

As one person said more succinctly on Facebook: “I am so healthy because I keep getting sick!” “I am so wealthy because I keep losing my money”. That sort of summarises things quite well.

And finally, Olga Basso, a perinatal epidemiologist, tried to highlight the absurdity of the immunity theft metaphor by tweeting: “Immunity debt is well-established, way back from 2021. Next year, we may have immunity inflation”.

Proposing a new metaphor: immunity robbery and theft

So, most experts seem to reject the immunity debt metaphor and the associated claim that a surge in respiratory viruses is the result of lockdown and children’s weakened immune systems.

They hypothesise instead that a rise in respiratory viruses in children may at least be partially due to them having been exposed to the coronavirus which has harmed their immune systems. To explain this, some different metaphors are used, as here in the article in the Global News, where the virus becomes a criminal:

“Emerging evidence suggests COVID-19 may be to blame, Furness says. ‘COVID, like many viruses, harms the immune system as part of its strategy,’ he said. For a virus to infect a host, it needs to either disguise itself or mutate to prevent the immune system from weeding it out. Or it needs to attack the immune system itself, he explained. What isn’t yet clear is how much damage COVID-19 may be doing and what long-term effects this could have, he said.”

In a tweet from 6 November Furness goes further and says about “the sudden rise in child hospitalizations for non-COVID illness. This can’t be explained by ‘immunity debt’ nor the tooth fairy, because neither exist. But it **CAN** be explained by immunity robbery, which we know DOES exist.” And on 8 November he defined immunity robbery as: “When a virus harms your immune response, making you more susceptible to subsequent infections”.

Ryan Gregory, an evolutionary biologist, coined a related metaphor that echoed that of immunity robbery: immunity theft. He talked about it on Twitter on 13 November (read this thread), but he is also quoted in a long article for Source, a New Mexican newspaper, from 23 November.

“’Adults shouldn’t be having children pay off the debts with viruses that put them in the hospital,’ said Anthony Leonardi, an immunologist and public health student at Johns Hopkins University. If we’re going to use this analogy, then what’s actually going on is ‘immunity theft,’ said Ryan Gregory, an evolutionary biologist at the University of Guelph in Canada. ‘COVID is robbing you of a functioning immune system, and so the more times they break in, the less you have in order to make use of to stop these infections,’ he said. “So over time, this could become a worse effect. I don’t know if that’s true, but I think it’s a very legitimate possibility, and that’s not something to dismiss, given the stakes.’”


Immunity debt is a concept that emerged in 2021 as a tentative hypothesis for explaining a surge in respiratory viruses in populations, where this makes relative sense, and in individuals, where this is not so much the case. The concept was picked up and amplified in social and traditional media, where it was used by some to support critical views of lockdowns. It also provoked critical views of the metaphor itself and the claims based on it. Tracing the metaphorical ramifications of that concept through some of the media, has given us a little insight into how metaphors are used to support and refute certain views about immunity.

The view that lockdowns, school closures and masking might have prevented children from strengthening their immune systems and therefore succumb to respiratory illness once lockdowns ended was contested through rejecting the framing of children as ‘currency’ used to clear the immunity debt. It was also contested through criticising the related metaphor of ‘the immune system is a muscle’ and by proposing alternative metaphors, such as ‘the immune system is old photographs’. On the other hand, the view that Covid might have damaged people’s immune systems and made them more susceptible to respiratory illnesses was supported by new metaphors of ‘immunity theft’ or ‘immunity robbery’.

In the discussions of immunity debt, one can see a split between those casting lockdown and masking as the villain or casting Covid as the villain. Narratives casting lockdown as a villain chime with talk about training one’s immunity, strengthening/weakening one’s immunity, given one’s immunity a work-out etc., talk that is scientifically tenuous and generally dissociated from using vaccination to help immunity. It’s not surprising then to see an online shop called ‘tonic’ telling people to start repaying their immunity debt by, for example, paying for daily vitamins.

It’s great to know about the metaphors floating around ‘immunity debt’ and apparent surges in respiratory viruses, but it’s even more important to get the science right. As Gregory says: “We still don’t know what exactly is happening. What is pretty clear, though, is that assuming that it’s simply ‘immunity debt’ after two years of no RSV infections is false in Canada. This is why we need to test multiple hypotheses and why we need data, not just-so stories.”


PS 4 December: Great summary overview of the ins and outs of ‘immunity debt’ on NPR – worth listening to, I think

Image: Pixabay



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