The COVID-19 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-19, they also had another an additional effect - a dramatic drop in cases of other illnesses like influenza (flu) and respiratory viruses.
However, once restrictions were lifted countries saw cases of flu surge dramatically. The phenomenon has been called the “immunity debt”, 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.
Published in Science Advances, a recent study, conducted by a team of researchers from NDORMS, University of Oxford and international collaborators highlights the significant changes in the transmission patterns of flu following the lifting of COVID-19 restrictions and supports the population immunity debt hypothesis.
The study warns 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. ‘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 minimise the impact of these on vulnerable populations, and (flu) vaccination should be encouraged among them.’
The researchers analysed global flu data from 116 countries between 2012 and 2024. They found that during the COVID-19 restriction periods, flu cases dropped by an average of 46% worldwide. However, in 2022, the first winter season after restrictions were lifted, flu cases surged by an average of 132% above pre-pandemic levels. Notably, South America experienced the most significant surge at 715%, and significant rises were also observed in populous nations, with China and the United States reporting increases of 222% and 157% respectively.
‘The degree of immunity debt was linked to how stringent a country's COVID-19 restrictions had been during the pandemic,’ said first author Li Chen, a visiting PhD student from Peking University. ‘Places that implemented the most severe lockdowns and social distancing measures tended to see the largest flu resurgences later on.’
Co-senior author of the study Junqing Xie, Postdoctoral Researcher in Pharmacoepidemiology and Pharmacogenetics at NDORMS said: ‘Our findings suggest that the risk of "immunity debt" should be on the radar of public health authorities in the development of future pandemic response strategies. Follow-up steps like a phased relaxation of any restrictions and promoting flu vaccination are advisable to alleviate potential adverse impact.’