Immunity debt Q&A

Answering some common queries about the immunity debt theory

A previous post looking at why children are currently unwell in summer with illnesses we only normally see in winter drew a lot of interest! Along with this came some really insightful questions, as well as a few misunderstandings.

I thought it would be helpful to address some of these in a specific Q&A post to help try and clarify what we do and don’t know.

Please read the original post first to understand the terms such as “immunity debt” and “overshoot”.

Are you sure it’s not due to Covid-19 or the Covid vaccines?

Yes, we are quite sure. As explained in the prior post, the displacement of respiratory viruses and unusually large waves which we associate with immunity debt occurred in countries which had experienced very little to no Covid-19 in their populations (Australia, New Zealand and Japan), and before Covid-19 vaccines were even licensed. What we are experiencing now is the continuation of this problem, and we can be confident neither Covid-19 not the Covid vaccines are to blame (especially since the majority of children affected are too young to even be eligible for vaccination).

Is it better for younger kids not to get infected until older?

One of the interesting questions is whether children who managed to delay infection from the winters during “lockdown” to a later summer or winter have benefitted from this delay. The simple answer is, at an individual level, yes they probably did. RSV infection is almost certainly more severe when encountered during the first year of life, so if you can last until you’re over 9-12 months until your first infection you are less likely to need hospital admission.

RSV infection at a young age is also associated with developing wheezing illnesses later in life, but the effect of the precise timing of this is unclear - it may also be beneficial to delay.

BUT - the issue here is not the effect at the individual level, but at the population level.

If you delay all of these infections, for the children who experience the delay, it may be a benefit. However, if this build up of immunity debt were to result in a larger wave later on with more overshoot, then potentially many more of the children aged <1y would be infected during this wave. Averaged over a few years, you could potentially end up with more infants aged <1 year infected with RSV due to delaying one season and building up a larger pool of susceptible children.

In this scenario, whilst the individuals who experienced the delay benefitted, the babies who came after suffered more.

Do kids need viral infections when younger?

There has been a lot of discussion about whether children’s immunity suffers more generally from a lack of exposure to viruses in early life, with many comparing this to the “hygiene hypothesis”, although this comparison is not quite correct.

For those unfamiliar, the hygiene hypothesis suggests that one of the reasons we have seen a large increase in allergic and other inflammatory diseases in recent years is due to reduced exposure to germs during early life, because hygiene has increased by such a large margin. This is not specifically about virulent pathogens (those that cause disease), or viruses such as RSV or Influenza, but about the normal, “friendly” bacteria which normally live on or inside us, known as the microbiome. As such, it is not relevant here.

Aside from this, whether it’s good to get mild respiratory viruses at a young age is actually more complicated than it would seem at first glance.

As a general rule, the fewer respiratory viruses you get during your life time, the better. Each one carries a small risk of something nasty happening, which are best avoided. As mentioned above, it is also definitely better to avoid many infections (including RSV and Influenza) during your first year of life when the risk of bad outcomes is highest.

However, there are some respiratory infections where getting them (for the first time at least) at a younger age is definitely much lower risk than getting them when you are older, such as chicken pox (the Varicella zoster virus) and now famously, Covid-19.

To add to the complexity, there actually are some generalised immune effects of exposure to live viruses/bacteria during infancy which can be beneficial - although this is not at all well understood. Off target immune effects (those which are not related to the infection which caused the immunity) have been demonstrated most convincingly to live, attenuated (meaning weakened) organisms included in vaccines, such as the BCG (a vaccine against TB containing a weakened form of Mycobacterium bovis) and vaccines containing the attenuated measles virus.

However, we cannot be sure that this can be generalised to live and virulent viruses which haven’t been weakened - for example, the virulent, wild measles virus does the complete opposite and can induce immune amnesia, erasing immunity to other pathogens which had been built up prior to infection.

We cannot therefore say there are no generalised immune benefits acquired from mild respiratory infections in childhood, but we definitely cannot say that there are benefits which exceed the harms or risks of getting infected.

Summary

Displacement of childhood winter illnesses into summer is not due to Covid-19 or Covid vaccines. There are infections which are best avoided during the first year of life, but displacing lots of these to a later date could potentially result in a wave which is much larger, eventually resulting in more infections in infants overall. There may be non-specific beneficial effects on immunity from mild respiratory infections in childhood, but it is unproven, and not clear that these would outweigh the risks which come with each infection. These are not related to the hygiene hypothesis.