2.Is it true that by getting fewer infections in 2020 and 2021 as a result of less mixing, adults and especially children have developed an “immunity debt”, causing them to get sick more often and worse now than would have been the case otherwise?

The term ‘immunity debt’ keeps being mentioned in the media as a supposed explanation for the surge in infectious illnesses. Let’s untangle this….

The term first appeared in scientific literature in August 2021, when a team of French scientists warned: “we may witness strong pediatric epidemic rebounds once personal protection measures are lifted.” The authors hypothesised that children had been exposed to less viruses and bacteria, and would be more prone to illness because of a “lack of immune stimulation”.  The term has been used widely ever since, generally by those who oppose any mitigations against Covid, especially in schools.

Although the term itself is new, the idea stems from what has been called the “hygiene hypothesis”, which was first proposed by David Strachan in 1989. Strachan’s theory was that the increase in allergies might be due to lower rates of infections in early childhood. However, such a link was never actually proven, and the theory came under growing criticism. According to the authors of a peer-reviewed article from 2016, babies and children acquiring a healthy ‘microbiome’, i.e. beneficial microorganisms that form part of all of our bodies, may well be vital for reducing risks of allergies, but this is an argument for supporting mothers to breastfeed, for enabling children to spend time outdoors, and for allowing all children to have a healthy diet. It is not an argument for exposing children to infections. 

Yet proponents of a so-called “immunity debt”, supposedly caused by children having been shielded from infections for several months early on in the Covid pandemic, seem to be reviving this previously debunked idea.