Flu 'freeze' reviewed
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New analysis finds flu cases dropped by about 95 per cent during COVID-19 pandemic, but air travel brought it back.
As COVID-19 spread, lockdowns, travel bans, and social distancing dramatically reduced cases of seasonal flu.
The study, led by researcher Zhiyuan Chen, sheds light on how the flu’s spread slowed down during strict COVID-19 interventions and how it eventually recovered, driven by the very same global travel networks that brought COVID-19 to nearly every corner of the planet.
It is a demonstration that “nonpharmaceutical interventions can be incredibly effective in disrupting viral transmission, pathogen diversity, and antigenic evolution, and are arguably more effective than vaccine efforts alone”, experts wrote in an editorial alongside the study.
During the pandemic’s early days, restrictions effectively “froze” the usual flu season.
With flights grounded and social distancing policies in place, fewer people were exposed to the flu - especially in regions with strict lockdowns.
However, tropical regions with looser restrictions, like parts of South Asia and West Asia, saw flu continue to circulate, allowing certain influenza lineages to survive and evolve.
These regions served as what the researchers call “phylogenetic trunk locations” for the virus, ensuring that influenza did not vanish entirely.
Interestingly, the study found that one flu lineage - influenza B/Yamagata - may have disappeared completely during the pandemic, as it has not been detected since.
By March 2023, as COVID-19 restrictions eased and international travel resumed, influenza activity surged back to familiar levels.
According to the study’s authors, the comeback highlights the flu’s remarkable resilience. “Influenza lineage dynamics are robust to prolonged disruption,” the report says, noting that international travel remains a key driver of global flu spread.
The full study is accessible here.
