A team of UK researchers has found evidence on how “immunity debt,” caused by extended periods of restrictions during Covid-19 pandemic, is causing changes in global transmission patterns of flu.
“Immunity debt” is a phenomenon where populations become more susceptible to infections after extended periods of reduced exposure. While the hypothesis was widely accepted, until now there has been little evidence to back up the theory.
The Covid pandemic led many countries around the world to implement strict public health measures like lockdowns, social distancing, mask-wearing, and travel restrictions.
While these steps were crucial and effective in controlling the spread of Covid, they led to a dramatic drop in cases of other illnesses like influenza (flu) and respiratory viruses.
However, post-Covid countries worldwide have seen a dramatic surge in cases of flu.
The researchers warned that this “immunity debt” could lead to major flu outbreaks in the years following a pandemic, as populations regain susceptibility to other viruses.
“Our findings provide evidence for the `immunity debt` hypothesis,” said senior author Daniel Prieto-Alhambra, Professor of Pharmaco- and Device Epidemiology at NDORMS, University of Oxford.
“By limiting flu exposure over the past few years, we`ve built up a more vulnerable population that is now seeing a major rebound in infections. Fortunately, strategies are available to minimize the impact of these on vulnerable populations, and (flu) vaccination should be encouraged among them,” he added.
The study, published in Advanced Science, analysed global flu data from 116 countries between 2012 and 2024.
They found that during the Covid restriction periods, flu cases dropped by an average of 46 per cent worldwide. However, in 2022, the first winter season after restrictions were lifted, flu cases surged by an average of 132 per cent above pre-pandemic levels.
Notably, the team found that the degree of immunity debt was linked to how stringent a country`s Covid-19 restrictions had been during the pandemic.
Countries with severe lockdowns and social distancing measures tended to see the largest flu resurgences later on.
The researchers urged public health authorities to factor in the risk of immunity debt in planning for future pandemic response strategies.
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