How did the COVID pandemic influence the seasonal flu?

The seasonal flu places a significant burden on healthcare systems globally, leading to over five million hospitalizations annually. 

Current influenza vaccine development relies on monitoring circulating strains that migrate across continents, requiring comprehensive surveillance. 

The COVID-19 pandemic’s unprecedented reduction in global travel provided a unique window for scientists to assess how restricted movement influenced the behavior and evolution of seasonal influenza.

In a new study, an international team of researchers from the University of Oxford, Fudan University, and KU Leuven analyzed data on the spread and genetics of seasonal influenza alongside international travel patterns. 

Focus of the research 

This study explored how long influenza viruses stayed within particular regions during restricted and unrestricted travel periods, shedding light on the diversity of flu strains before, during, and after the pandemic.

“During the COVID-19 pandemic, nonpharmaceutical interventions were introduced worldwide, which led to human behavioral changes on an unprecedented scale,” noted the study authors.

“This led to a decline in the global prevalence of endemic respiratory pathogens, including seasonal influenza subtypes H1N1pdm09 and H3N2 and lineages B/Victoria and B/Yamagata.”

Reduced influenza levels and diversity

When COVID-19 pandemic-related travel restrictions were enforced worldwide, cases of seasonal influenza declined sharply due to less movement and fewer social interactions. 

Yet, influenza levels rebounded rapidly as international travel resumed. The virus, despite its temporary dip in cases, was maintained in circulation and continued to evolve genetically.

“It was remarkable how quickly seasonal flu re-established to a pre-pandemic equilibrium just a few years after the height of the COVID-19 pandemic,” said lead author Zhiyuan Chen, a scientist at the University of Oxford and Fudan University.

This bounce-back revealed that, although COVID-19 measures temporarily slowed influenza spread, the virus was preserved within certain regions and resumed its typical patterns once movement restrictions eased.

Climate variations and flu transmission

Tropical regions, particularly in South and East Asia, support year-round flu transmission due to their climates, which fosters a greater diversity of influenza strains. 

During the pandemic, increased genomic surveillance capacity enabled scientists to better understand the role of various regions, including Africa and West Asia, in flu’s global circulation. 

These areas, with fewer restrictions and relatively low COVID-19 spread, contributed to sustained flu transmission.

“Increased genomic surveillance capacity established during the COVID-19 pandemic means that we are finally getting a deeper insight into the global distribution patterns of seasonal flu and other respiratory viruses,” explained Kraemer. 

Climate, co-circulating viruses, and human behavior

Study co-author Moritz Kraemer is an expert in the Department of Biology and the Pandemic Sciences Institute at the University of Oxford. 

“These novel and large openly accessible datasets provide an opportunity to learn about the intricate relationships of climate, co-circulating viruses, and human behavior.”

With improved global genomic surveillance now in place, scientists have a new opportunity to track the evolution of seasonal flu and its global migration patterns. 

This deeper understanding may reduce the risk of vaccine mismatches, making it possible to tailor interventions and vaccines more effectively, which could ultimately alleviate the healthcare burden of seasonal influenza. 

Climate change and flu transmission 

The goal becomes increasingly relevant as climate change makes more regions suitable for flu transmission throughout the year.

“Further efforts should still focus on the continuing surveillance of seasonal influenza viruses and other respiratory pathogens, particularly in resource-limited regions,” noted Hongjie Yu from Fudan University. 

“The established surveillance systems for seasonal respiratory pathogens could also play an extremely vital role when the next pandemic emerges in the future.”

Understanding respiratory viruses 

As climate conditions shift and the possibility of year-round influenza transmission in previously unsuitable areas grows, continued focus on influenza surveillance and response remains essential. 

Increased monitoring in regions with limited resources, combined with these new datasets, equips health systems to respond proactively. 

This progress in understanding the interplay between climate, global travel, and respiratory viruses may prove instrumental in managing both seasonal influenza and future pandemics.

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