Study Links Global Surge in Flu Cases to COVID-Induced Immunity Debt

New Delhi: A new study led by researchers from the University of Oxford has found compelling evidence that "immunity debt"—a reduced immune response caused by prolonged lack of exposure to common pathogens during the COVID-19 pandemic—is contributing to a global resurgence in flu cases.

The study, published in Advanced Science, analysed influenza data from 116 countries spanning 2012 to 2024. It revealed that during the height of COVID-19 restrictions—marked by widespread lockdowns, social distancing, mask mandates, and travel bans—flu cases fell sharply, with an average global decline of 46%. However, after these measures were lifted, flu infections surged by an average of 132% above pre-pandemic levels during the 2022 winter season.

The researchers attribute this rebound to "immunity debt," a phenomenon where reduced exposure to viruses during public health lockdowns leads to a decline in population-level immunity, making people more susceptible to infections once restrictions are eased.

Professor Daniel Prieto-Alhambra, senior author of the study and expert in pharmaco-epidemiology at Oxford's Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences (NDORMS), explained that the findings confirm long-standing assumptions about immunity debt. “By limiting flu exposure over the past few years, we've built up a more vulnerable population that is now experiencing a sharp increase in infections,” he said.

The study also found a direct correlation between the severity of a country’s COVID-19 restrictions and the size of its post-pandemic flu resurgence. Nations that implemented stricter containment measures saw more significant increases in flu activity, highlighting how stringent lockdowns contributed to the loss of natural immunity against seasonal influenza.

While the decline in flu during COVID-19 restrictions had temporary benefits, such as reduced flu-related hospitalizations, the researchers caution that the resulting immunity gap poses long-term challenges.

They recommend that public health authorities account for immunity debt when designing future pandemic strategies and stress the importance of promoting influenza vaccination, especially among high-risk groups, to mitigate the impact of future outbreaks.