Lockdowns put us at the mercy of disease

By disrupting our finely balanced ecology, we made the return of sickness more aggressive this winter

Fans of Little Women will know that Beth March died of the lingering complications of scarlet fever, but who would have thought that this bacterial disease would be in the headlines in 2022? Is this because we have left children who were born during or just ahead of the Covid pandemic with an “immunity debt”?

It is now widely acknowledged that lockdowns caused harm to our already stretched health service, with many of the direct consequences such as increased cancer and cardiovascular deaths being reported regularly. Most of these harms were entirely predictable. Less obvious was how the more indirect consequences of lockdown might play out, such as the effect on our relationship with other widespread pathogens.

I am used to viewing infectious disease from an ecological perspective. It did not come as much of a surprise to me that some noncovid seasonal respiratory diseases took a knock on the head during lockdown. Many took this to be an indication that lockdowns were working to stop the spread of disease, forgetting that the impact of lockdowns on already established or endemic diseases is completely different to the impact on a new disease in its epidemic phase.

It is worth explaining this. For an individual, immunity debt can be interpreted as a gap in the level of protection that you might be expected to have from previous exposure to the disease in question. The same principle also applies to a whole population. This is because there is a threshold of immunity at which rates of new infections start to decline – known as the herd immunity threshold. If we are below this threshold, we are in immunity debt.

With endemic diseases, we go into immunity debt as winter sets in. This causes a seasonal increase in infection and leaves us in “immunity credit” for the rest of the season. Over the summer, the numbers immune fall, leaving us again with an immunity debt in the winter. However, any small change to the transmissibility of a pathogen (perhaps by locking down society) will disrupt the rhythm.

Thus, a pathogen entering an immunologically naïve population will start off with a massive immunity debt, leading to infections growing very rapidly at this epidemic stage. When lockdowns are eased, not only do endemic diseases quickly reestablish themselves, they can – as we have seen – return more aggressively than usual on account of the immunity debt they have amassed in the interim.

This can cause all sorts of problems. Health-care systems will have to be prepared for higher than usual hospitalisations. It is a particularly troublesome task for the NHS, which continues to struggle with capacity.

Furthermore, the synchronised rise in these suppressed infections enhances the possibility of coinfection; this has been recognised as a potential cause of a spate of adenovirus infection-related deaths earlier this year. And if the likelihood of clinical complications increases with age, there will be obvious perverse consequences of delaying infection.

It is hard to say which of these potential mechanisms is the key contributor to the very unfortunate re-emergence of scarlet fever as a cause of severe disease and death in young children in the UK. Group A Streptococcus, its causative agent, exists within a complex network of other bacterial species which also may have suffered changes in composition as result of lockdowns. Disturbing 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 perturbation in our finely balanced ecological relationship with the organisms, which are capable of causing serious disease.

Eventually, that balance will return. The immunity debt that we have incurred will be gruesomely paid off and scarlet fever will once again become a storybook word.

Sadly, the same cannot be said of the enormous financial debt we have taken on board to pay for these fruitless lockdowns. Our children will be shouldering this debt for years to come.