Hello, and welcome back to Everyday Science.
This week, there is increasing attention on a surge of pneumonia cases in China. If anyone is worried that this sounds like the start of the Covid pandemic five years ago, they shouldn’t be. These cases are due to a known bug called human metapneumovirus (HMPV), and there are no unusual features to the outbreak, the World Health Organisation has said.
But there has been a rise in HMPV cases in China. With this being just the latest in a line of other infectious disease spikes in the past few years around the world – including flu and respiratory syncytial virus (RSV) – some scientists have expressed concerns that Covid has damaged people’s immune systems.
In fact, the recent pattern of unusual disease outbreaks probably is related to Covid – but not in the way you might think.
The most likely explanation is what is sometimes called immunity debt. This is the idea that the lockdowns and reduced social mixing caused by Covid led to lower population immunity to many pathogens. When socialising returned to normal, this “debt” was repaid by higher-than-usual rates of illness.
To understand why this makes sense, we need to consider the normal patterns of infectious disease, before Covid struck.
Many viruses and bacteria are constantly circulating in populations. A new infection should cause you to become immune but this fades over time and babies with no immunity are being born all the time. Plus, the germ itself can mutate, letting it overcome immunity.
For many bugs, something about winter weather tips the advantage to the pathogen, which sparks a wave of infections. But this leads to higher population immunity; now there are fewer people available to infect, infection rates fall again.
This is why we usually see regular winter spikes in bugs like flu, RSV and HMPV, although the exact timing depends on the pathogen, and probably other things we don’t yet understand.
Covid arrives
In 2020, Covid spread around the world and many countries responded with lockdowns. Even when these had offically ended, there was less social mixing and therefore less spread of germs for more than two years.
As a result, in the UK’s first winter lockdown of 2020-21, there were very few flu cases. Even in the second year, flu rates were low.
By the winter of 2022-2023, when socialising was almost back to normal, health officials warned they expected a big surge of flu cases. And they were right, although it still wasn’t as high as some previous years. There has always been some variation in flu season severity from year to year.
But this was not the only effect. The weirdness had actually begun in the preceding summer of 2022, when doctors saw a surge in cases of RSV a few months after all official Covid restrictions ended.
RSV usually causes mild illness in babies and young children, but can become severe in some, and send them to hospital. Paediatricians remarked on how unusual it was to see this spike in what is usually called a winter illness. It only made sense in the light of immunity debt, said Dr Alasdair Munro, a paediatrician at University Hospital Southampton.
Worse was to come. In the winter of 2022-2023, there was also an unusual pattern of infections with a bacterium called Group A streptococcus.
This normally causes sore throats and other mild illnesses, but can occasionally lead to more serious invasive infections. That winter there were hundreds of deaths caused by such cases, including among children.
Parents were warned to watch out for symptoms, and to lower their bar for seeking medical help if their children became ill, leading to pressure on GP and hospital services. “The Group A strep surge in December 2022 caused some of the worst pressures children’s A&E has ever seen in this country, not just because of the infections Group A strep was causing, but because a lot of people were understandably very worried,” said Dr Munro.
“With the usual fevers and coughs and colds that parents might have felt comfortable waiting at home with, people were coming very early to A&E because they were so worried about what was on the news.”
HMPV
It is too soon to say, but it is possible that the current wave of HMPV in China is also due to a rebound of infections after immunity debt caused by Covid lockdowns.
A sceptic might argue that lockdowns happened too long ago to still be affecting disease patterns today.
But the lockdowns in China were at times stricter than elsewhere and went on for longer – until the end of 2022, nearly two years after the UK stopped strict official lockdown. So the country could be more vulnerable to more severe disease outbreaks, said Dr Connor Bamford a virologist at Queen’s University Belfast.
Nevertheless, the idea of immunity debt has been controversial. When it first gained currency, in 2022, a writer for the Financial Times called it a “misguided and dangerous concept”, saying it suggests that lockdowns were harmful.
Today, it still meets resistance from those who argue for greater caution around preventing Covid.
Professor Stephen Griffin at the University of Leeds, who is chair of the Independent Scientific Advisory Group for Emergencies, said the term immunity debt suggests we shouldn’t take basic hygiene precautions to reduce common infections.
“There is a prevailing narrative of ‘We have to experience these illnesses. It’s something that we have to endure to make our immune system stronger,’” he said. “That’s absolute crap.”
But even if some dislike the term itself, the idea that reduced mixing during the pandemic led to subsequent surges in infections is now uncontroversial, said Dr Munro, who has recently published a paper making this case.
This explanation was also supported in a study of the Group A strep spike, by scientists at the UK Health Security Agency, and a recent analysis of surges in various respiratory infections in 13 European countries.
Fortunately, the immunity debt explanation is good news. If immune systems had been permanently damaged by Covid on a population scale, we would expect rates of other infections, like flu and RSV to get worse and worse every year. And this doesn’t seem to be happening.
So, while health officials clearly need to keep an eye on any unexpected infection outbreaks, like the HMPV surge in China, for now, it does not seem a reason to panic.
Other things I’ve written recently
You may have seen headlines elsewhere saying that new research shows we should only drink coffee in the morning, to get any health benefits for the heart. In fact, the research has been overstated, as I explain here.
I’ve been watching
I was a big fan of the first two series of The Traitors, and the new series, which began on 1 January, is shaping up to be just as good so far.
If you want to know what science has to say about deception detection, then look no further than this explainer piece, here.
