Setting the record straight on the term ‘immunity debt’

Rates of respiratory syncytial virus (RSV) infection continue to rise, especially among children. Together with a seasonal increase in influenza infection, it means the non-Covid infectious disease “holiday” of the last two winters is definitely over.

In most cases, RSV causes a mild illness with symptoms similar to that of a regular cold. But it can also cause lung infections, such as bronchiolitis or pneumonia, especially in infants, premature babies and children with underlying health problems.

RSV is the commonest cause of lower respiratory tract infection in children younger than five years. Pre-Covid, a meta-analysis estimated that, worldwide, RSV was associated with 3.6 million hospitalisations annually. And before the pandemic, RSV was highly seasonal in temperate countries like Ireland.

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Now, spikes in RSV infection have developed as non-seasonal events. One explanation for out-of-season RSV resurgences is decreased population immunity following a prolonged period of minimal RSV exposure. This difference between expected immunity levels in a population and the immunity levels we are seeing now has been labelled “immunity debt”.

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Despite a somewhat shaky scientific foundation, the term “immunity debt” is circulating widely on social media. On Twitter, the term has taken on a life of its own. Some people have taken it to mean that a lack of exposure to bugs such as RSV and influenza has irrevocably damaged the immune system.

In August, 2021, scientists in France were the first to coin the term “immunity debt” to describe a reduction in population-level immunity. However, they did so in a position paper and without scientific evidence to back up their assertion. Many infectious disease experts now say the term and its surrounding narrative are “dangerous” and can promote Covid-19 “misinformation”.

A worrying interpretation of “immunity debt” – and one that has been promoted by those with a disinformation motive – suggests people’s immune systems are weaker now, due to a lack of exposure to viruses while observing Covid-19 public health measures over the last 2½ years. But for this to happen, our immune systems would have to resemble a muscle that atrophies with lack of use. There is no evidence that our immunity weakens on its own.

The ambiguity of the phrase “immunity debt”, however, means that there is another, more relevant interpretation of it. It’s not just RSV that is putting children in hospital but respiratory enteroviruses, influenza and parainfluenza as well. So, 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.

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However, this interpretation is being used online and in the media to blame Covid-19 public health measures for the current surge in infections. Lockdown, masking and social isolation delayed children’s exposure to respiratory illnesses for more than two years. Ergo, they cause “immunity debt”.

But experts are adamant that blaming Covid-19 public health measures, such as masking and distancing, on the current surge in respiratory illness is not only false, it promotes the notion these measures should be shunned as a future pandemic response.

Some scientists have suggested that the surge in RSV hospitalisations might be the result of SARS-CoV-2 infection causing immune deficiencies that leave people more susceptible to other infections. After all, Covid, like many viruses, harms the immune system as part of its strategy to gain entry.

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. What isn’t clear, but is the subject of ongoing research, is how much damage Covid-19 may be doing to our immunity and what long-term effects this could have.

Which is a completely different hypothesis to the online gaslighting around “immunity debt”.