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The reason why lockdown may have left a generation with an ‘immunity debt’

Winter bugs like Strep A are spreading
Winter bugs like Strep A are spreading

It is a parent’s nightmare - an illness that appears banal, making children feel a bit under the weather, but which in rare cases can lurch swiftly from routine to critical. Once it becomes invasive, entering the bloodstream, Strep A bacteria can lead to sepsis, chest, bone and joint infections, even necrotising fasciitis, otherwise known as the “flesh eating disease”.

Doctors are reporting a higher than usual number of cases. Nine children have now died due to complications from Strep A, and the level of concern is such that the government is considering giving preventative antibiotics to pupils at schools with cases.

It is not the only bug thriving this winter: latest monitoring shows that 10.5 per cent of those swabbed now test positive for flu, with the rate in 5-14 year olds highest, at 18.5 per cent. This time last year, by contrast, in the weekly Flu and Covid report, a mere 0.6 per cent of those tested came back positive. Flu hospitalisations in young children are nearly 20 times as high as last year. Cases of respiratory syncytial virus (RSV) - which can cause bronchiolitis and breathing problems, especially in toddlers - are also soaring here and abroad. In Germany, intensive care doctors have warned that hospital paediatric units are stretched to breaking point in part due to the rise in RSV.

There is an obvious cause of these increased rates: our first winter in three without lockdown. Dr Susan Hopkins, the UK Health Security Agency’s chief medical adviser, noted this week that “we’re back to normal social mixing… as things get back to normal, these traditional infections that we’ve seen for many years are circulating at great levels.”

It is an uncontroversial conclusion. Renewed mixing, renewed infection. More infection, more chance for the odd case to become grim. What is more controversial is the notion of ‘immunity debt’: under-exposure to bugs during the distancing and lockdowns of Covid that have left people, specifically children, more vulnerable now.

Have Covid lockdowns lowered our immune defences?

The Streptococcus Pyogenes bacteria usually cause mild infections but can result in a potentially deadly disease - Science Photo Library RF
The Streptococcus Pyogenes bacteria usually cause mild infections but can result in a potentially deadly disease - Science Photo Library RF

“Potentially,” says Shiranee Sriskandan, professor of infectious diseases at Imperial College London. “Children normally catch scarlet fever in their first year at school, if at all. We know that scarlet fever rates plummeted during 2020-2021. We therefore think that school-aged children may not have built up immunity to Strep A, and so we now have a much larger cohort of non-immune children where Strep A can circulate and cause infection. This is coupled with an unexpected rise in Strep A infections at the wrong time of the year, when winter viruses like RSV are circulating.”

Hopkins said the UK Health Security Agency is now “exploring” whether lower-than-normal levels of immunity may be contributing to infection numbers and severity. Scientists divide bugs into those which - like chicken pox - the immune system can repel forever after one infection, and those - like the common cold - which have ways of evading the immune system and therefore require constant low-level exposure to ensure a degree of protection. Without that low-level exposure, some bugs are finding more unusually unprotected human hosts to invade.

Even after two years of lockdowns, most adults’ immune systems have at least some memory of these bugs, from before Covid. But there is a particular problem with young children, who, because of Covid restrictions, have reached school age with little or no exposure to bugs they would normally have encountered, and now find themselves surrounded by them in high doses.

“They’re the kids that have very little background immunity because they were all babies or toddlers in Covid,” says one consultant in infectious diseases, who asked to remain anonymous. “They just haven’t built their immune memory banks up. Rather than getting their exposure to viral infections such as coughs and colds, as well as other more serious infections like group A Strep over two or three years, they’re getting them concentrated into one winter.”

A picture shows swings tied up in a children's playground in the Highfields area of Leicester, central England on July 17, 2020 - AFP
A picture shows swings tied up in a children's playground in the Highfields area of Leicester, central England on July 17, 2020 - AFP

How worried should we be about immunity debt? Though it’s true that children are just getting the illnesses they would ordinarily have had in the last two years, more of them are getting it at the same time - and the unwinding of Covid restrictions has disrupted the “seasonality” of bugs. The rise in Strep A is usually seen in the spring. Such waves could put more pressure on already stretched health services.

The British paediatrician Alasdair Monroe has said the pandemic means we now have reduced immunity at population level, so it will take more people to become infected than normal to achieve herd immunity. That could mean more total infections of diseases such as flu, Strep A and RSV than normal this season. “You are not just repaying the immunity debt (i.e. catching up on infections which were previously missed), but you are paying considerable interest on it too (extra infections caused by a bigger overshoot),” he wrote in a blog.

All this means it is more important than ever to be up to date with vaccines. Uptake of the flu vaccine among children aged two to three is down by nine per cent on last year.  Common sense measures, particularly over hygiene and hand washing, will help families to limit infections, the consultant says. “Keep wounds clean. Don’t cough or be coughed at by people. I certainly wouldn’t say that everyone has to go mad.”

Experts stress that serious complications from Strep A remain exceptionally rare, and encourage parents and caregivers to know the signs for when to seek medical help.

The consultant says immunity debt is a temporary result of the lockdowns: “There was always going to be this issue emerging from Covid restrictions. The good news is that next winter we should be back to normal.”