fans of Little woman You’ll know that Beth March died of lingering complications from scarlet fever, but who would have thought that this bacterial disease would be hitting the headlines in 2022? Is this because we have left children who were born during or just before the Covid pandemic with an “immunity debt”?
It is now widely recognized that blockades caused damage to our already stretched health service, with many of the direct consequences, such as increased cancer and cardiovascular deaths, being regularly reported. Most of this damage was completely predictable. Less obvious was how some of the more indirect consequences of the lockdown might play out, such as the effect on our relationship with other pathogens circulating within our communities.
I am used to seeing infectious diseases from an ecological perspective. So I wasn’t so surprised that some non-Covid seasonal respiratory illnesses almost immediately started getting hit on the head during lockdown. Many took this as an indication that the lockdowns were working to stop the spread of disease, forgetting that the impact of lockdowns on already established or “endemic” diseases is entirely different than the impact on a new disease in its “epidemic” phase. .
This is worth explaining. For an individual, “immunity debt” can be interpreted as a gap in the level of protection that could be expected from prior exposure to the disease in question. The same principle also applies to an entire population. This is because there is a population immunity threshold at which the rates of new infections begin to decline, known as the herd immunity threshold. If we are below this threshold, we are in debt of immunity; if we’re above, we’re on credit, at least for a while.
With endemic diseases, we go into immunity debt as winter sets in and the herd immunity threshold (which is determined by the transmissibility of the pathogen) rises. This causes a seasonal spike in infection and leaves us with credit for the rest of the season. During the summer, immune numbers drop, again leaving us with an “immunity debt” in the winter. Any small change in pathogen transmissibility will disrupt the rhythm and can make these pathogens disappear by lowering the Herd Immunity Threshold and temporarily canceling their normal “immunity debt”.
A pathogen entering an immunologically naive population will start with a massive “immunity debt”, leading to infections growing very rapidly at this “epidemic” stage. This is why lockdowns hardly make a dent in the progress of an epidemic, but can have such a significant effect on endemic diseases.
Such effects are, however, transitory. Endemic diseases will soon reestablish themselves, and as we have seen, they may come back more aggressively than usual due to the “immunity debt” they have accumulated in the interim.
This can cause all kinds of problems. Naturally, health systems will need to be prepared for higher-than-usual hospitalizations during this readjustment period. It is a particularly problematic task for the NHS, which continues to struggle with capacity issues.
Furthermore, the synchronized increase in these suppressed infections increases the possibility of coinfection; this has been recognized as a potential cause of a series of deaths related to adenovirus infection earlier this year. And if the probability of clinical complications increases with age, there will be obvious perverse consequences of delaying infection.
It is difficult to say which of these possible mechanisms is the key factor contributing to the very unfortunate resurgence of scarlet fever as a cause of serious illness and death in young children in the UK. Group A strep, its causative agent, exists within a complex network of other bacterial species that may also have undergone compositional changes as a result of the Covid lockdowns. Disrupting this order can have a profound impact on an individual’s ability to resist disease.
More than anything, it is clear that we are experiencing an entirely predictable disturbance in our finely balanced ecological relationship with organisms that are capable of causing serious disease.
Eventually that balance will come back. The “immunity debt” we have incurred will be horribly paid off and scarlet fever will once again become a storybook word. Unfortunately, the same cannot be said for the enormous financial debt we have taken on to pay for these unsuccessful blockades. Our children will assume this debt for years to come.
Sunetra Gupta is Professor of Theoretical Epidemiology in the Department of Zoology at the University of Oxford.
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