My oldest daughter informed me last week after school: “Mom, only five people turned up to class next door today — half of ours didn't show either." This has already happened a few times earlier this year. My younger daughter is at elementary school, and came back distressed — so many of her classmates were down with fever that she was again shunted into other quarantined classes. Theirs was quarantined for an entire week, with no outdoor PE nor cafeteria allowed.

Things are not much better for adults. Last Friday, seven health-care workers were already off sick at my centre and patients continued to flock in — it's been a perfect storm.

What Does “Immune Debt” Mean?

So you may ask: why is the seasonal flu so bad this year? Here’s a breakdown.

One Lancet study showed that respiratory syncytial virus (RSV) infections among UK children plunged by 99.5% in the winter of 2020–2021. RSV normally causes annual outbreaks worldwide in kids, and it almost disappeared. This means British children hardly caught colds, bronchitis or pneumonia over two winters.

Because COVID-19 lockdowns stopped all pathogens from spreading. Since around 2020, viral/bacterial infections in children globally reduced to almost zero for some — such as bronchiolitis, chickenpox, measles, pertussis, pneumococcus and meningitis.

The reduced pathogen circulation, and delayed vaccinations, 'spoiled' populations immunologically by depriving them of immune stimulation. This susceptibility is referred to as "immune debt" or the "immune gap." When restrictions were lifted, pathogens came roaring back; immune systems had to "pay their dues."

Why Does Reduced Pathogen Exposure Increase Susceptibility?

The study from Soochow University evaluated blood samples taken in 2018–2019 from 368 healthy children, comparing them with 368 matched samples collected during 2021–2023. By 2023, RSV antibody levels in 1–4 year olds had fallen so steeply they provided no protection against the virus — a stark pre-pandemic comparison.

That is why children in winter 2023 felt the brunt of RSV infections: their defences waned. Schools & hospitals bear that load now, immune systems trying to "catch up."

Some Practical Measures

1. Rebuild Immunity: Gradually increase outdoor time to restore natural defenses.

2. Vaccination Catch-up: Get missed jabs like flu, pneumococcal.

3. Hygiene Maintain: Don't over sanitize; let microbes train your immunity with moderate exposure.

4. Symptom Watch: Catch RSV/flu early to avoid outbreaks.