What is "immunity debt"?

A new study sheds further light on this seemingly controversial issue

One of the most noteworthy phenomena since the Covid-19 pandemic has been the erratic, sometimes explosive recurrence of seasonal infections such as RSV and Group A Strep. Many people have attributed this phenomenon to something termed “immunity debt”, sparking a huge amount of debate online and even in the media as to what this means. Some outlets even referred to the idea as “misguided and dangerous”.

My colleague Thomas House and I recently published an article on the issue in the journal “Clinical Infectious Diseases”, and a brand new publication has shone some new light on the topic. I thought this would be an excellent opportunity to explain this concept and clear up some of the confusion and controversy around it.

Normal seasonal pathogen circulation

Some pathogens such as RSV and Group A Strep are “endemic”, which broadly means they are always circulating to some extent. They are also seasonal, meaning rather than circulating at a constant level, they come and go in annual waves. There are several reasons this happens, however one of the most important is levels of population immunity.

During a wave of infections, exposure to these pathogens causes a boost to peoples immunity. This includes people who were last exposed a while ago, or to young children who have never been exposed before. This causes the level of population immunity to rise. Once it’s high enough, it becomes too difficult for the pathogen to continue to circulate and the wave of infections rescinds.

After the next few months, population immunity gradually falls again and new children are born who have never been exposed. Eventually, population immunity is low enough for another wave of infections to occur. This occurs in a relatively predictable pattern, so there is a rough range of “normal” levels of immunity.

What happened during the pandemic?

Measures used to prevent the circulation of Covid-19 had a predictable impact on also preventing the circulation of other infections. In fact, it was so successful in some cases we think it actually caused the extinction of an entire strain of influenza (the Yamagata strain). As seasonal pathogens did not circulate at their usual time, this causes population immunity to fall lower than it usually would. This is because immunity falls over time since the last exposure, and more children than usual had been born who had never been exposed.

We knew this would happen, and it was predicted that this would cause disruption to the circulation of these pathogens when they recurred.

What is immunity debt?

Immunity debt is simply the term derived to explain population immunity being lower than what would be normal for a particular pathogen. The term “debt” refers to an amount of additional immunity needed to be accrued to cause the seasonal wave to decrease compared to normal, as it is starting from a lower baseline. This is what would explain the larger than usual number of infections observed for the recurrence of pathogens such as RSV or Group A Strep; or even the current epidemic of Mycoplasma pneumoniae being observed in North America (much of the rest of the world experienced this in winter 2023/24).

Some people mistook the term to mean a general phenomenon where the immune system needs constant exposure to a variety of pathogens in order to be “strong”. This is not what it means. It is about immunity to specific pathogens which usually occur in cycles. It has never been referred to in this way in the academic literature or by any specialists in infectious diseases or immunology.

Is Covid-19 impacting immunity?

A small community of people have claimed that these increased waves of infection are happening due to Covid-19 causing immunodeficiency in children and adults. Some have even gone so far as to refer to Covid-19 as “airborne AIDS”. This is completely false. There is no evidence of Covid-19 causing any clinically relevant changes to the immune system beyond that which is seen with all viral infections during the acute infection. I have previously written about this disastrously wrong theory which you can read below.

What’s the new research?

An excellent new study from a paediatric emergency medicine network has provided more evidence towards the theory of immunity debt. They tracked the levels of several seasonal infections during and after the pandemic, looking for a “dose response” relationship between how much each infection was suppressed during the pandemic and how great the level of the infection was when it recurred. As per the theory of immunity debt, we would expect that pathogens which were suppressed more to have a higher rebound in infections. If there was any problem with immunity due to Covid-19 infections, there is no reason why this would happen and the recurrence of infections would have no reason to be proportional to their suppression.

It should be no surprise that we observed precisely what we would expect based on our existing knowledge of infectious disease transmission and population immunity. There was a clear dose response relationship between infection suppression and recurrence, during and after the pandemic respectively. Such a correlation was not observed for urinary tract infections, used as a control as they would not be expected to be impacted by non-pharmaceutical interventions and therefore by immunity debt. This provides excellent evidence for the theory of immunity debt as the explanation for the phenomenon we have observed after the pandemic with the circulation of seasonal infectious diseases.

Hospital presentations of several infectious diseases as their actual (black), modelled (red), and expected (purple) rates.

Furthermore, all the pathogens which had unusual surges post-pandemic so far have already settled back into their normal circulation patterns. RSV and Group A Strep are now behaving as normal. This is again what we would expect according to the theory of immunity debt, and is not what would happen if Covid-19 was impacting children’s immunity.

Summary

Immunity debt describes the lower than normal levels of population immunity to seasonal pathogens which occurred after the pandemic due to the suppression of these pathogens as a side effect of the interventions to suppress Covid-19. This was expected prior to the pandemic, and has happened according to what we would expect. A new study has demonstrated a dose-response relationship to infection suppression and recurrence which provides further evidence towards this theory. There is no clinical or epidemiological evidence of Covid-19 causing any relevant immunodeficiency in the population.

For a more technical and detailed discussion you can read the open access version of my publication with Thomas House here.