What is immunity debt – and is it really making kids sick?

There’s been an uptick in viral infections in the post-pandemic period, but that isn’t necessarily a bad thing.

The COVID-19 pandemic may be in the rearview mirror, but Europe is still grappling with its aftereffects.
In Denmark, for example, the number of children and teenagers with mycoplasma pneumoniae – a bacteria that causes respiratory tract infections – surged about three-fold in the 2023-2024 season compared to the pre-pandemic years. Hospitalisations were also 2.6 times higher.
However, the Danish kids’ infections were no more severe than in previous years – indicating that while more people got sick, they didn’t get sicker than expected.
Countries like England, Germany, and France have also seen unusual upticks in infections such as respiratory syncytial virus (RSV) in recent years.
“There has been quite a bounceback in a number of these infections which were not circulating significantly for a good winter or two, and they came back with quite a vengeance,” Dr Peter Openshaw, a respiratory doctor and expert on the flu and RSV from Imperial College London, told Euronews Health.
That’s because Europe is likely still paying off an “immunity debt” due to the drop-off in infections during the COVID-19 pandemic.
But what exactly is immunity debt – and how worried should parents be heading into winter?
In the early pandemic period, non-COVID illnesses fell in areas where people stayed home, avoided other people, washed their hands frequently, and took other steps to prevent themselves from being exposed to viruses.
Several viruses, like some strands of the flu, were totally wiped out during that time, but others simply came back once restrictions lifted and people began socialising more often – a phenomenon that some scientists refer to as an “immunity debt” that is still being paid off.
“Decreasing burdens on hospitals during the height of COVID, it was good to go in debt for that,” Dr Amesh Adalja, an infectious disease doctor and senior scholar at the Johns Hopkins Center for Health Security in the US, told Euronews Health.
The term “immunity debt” is sometimes used to suggest that natural infections are better for our immune systems than vaccines and that pandemic-era restrictions were ineffective because people still got sick later on.
But those ideas are “manifestly wrong,” Openshaw said, adding that public health restrictions “saved many, many thousands, possibly millions of lives”.
Instead, the post-pandemic increase in infections was largely inevitable, researchers say.
RSV is a prime example. It’s a common virus that spreads through close contact and typically causes mild, cold-like symptoms. Most toddlers get it by the time they are two.
But as babies were born and kept away from others during the pandemic, the group of people who had never been exposed to the virus grew, creating an opportunity for spread once restrictions were lifted.
That might not be a bad thing, given RSV is much more dangerous for babies under six months old.
“Each human owed their debt to RSV, and it just got delayed. The debt collector was coming,” Adalja said.
“The idea that somehow people were going to escape these respiratory viruses forever – these endemic respiratory viruses – that’s false”.
While the immunity debt dynamics hold true for many viruses, some of them may be seeing a resurgence due to reasons other than immunity debt.
For example, Adalja said that the rise in whooping cough cases, also known as pertussis, is likely cyclical.
There are typically whooping cough outbreaks every three to five years, though France reported its worst outbreak in 25 years earlier this year.
In 2021, European paediatric societies called for strengthened childhood vaccination programmes to counter immunity debt, and since 2023, RSV vaccines have been available for pregnant women and older adults in the European Union. They rolled out this year in the UK.
“Vaccines are a great way of filling the gap in immune stimulation which was left by a reduced rate of virus circulation,” Openshaw said.
Notably, increases in illnesses caused by immunity debt – or cyclical surges – could pose a bigger problem for health systems if they are hit with too many patients at once.
Many are struggling with shortages of healthcare workers and have little capacity to handle surges of patients with common childhood infections and pneumonia among adults.
“We’re still seeing an awful lot of hospital attendances, serious illness with these viruses because they’re circulating at a higher-than-previous level,” Openshaw said.