An international team of researchers, including researchers from the Oxford Martin Programme on Pandemic Genomics, have traced the global movement and evolution of seasonal influenza viruses to evaluate how the virus is impacted during pandemics.

In the study published in Science, the researchers combined data on the spread of seasonal flu, its genetic makeup, and international travel patterns to study how the viruses moved and evolved. This approach helped to estimate how long the viruses remained in certain regions during periods of high and low volumes of international travel and how their genetic diversity changed before, during, and after the COVID-19 pandemic.

Seasonal flu epidemics place a heavy strain on healthcare systems, leading to over 5 million adult hospitalisations annually. Developing effective vaccines relies on tracking flu strains as they spread globally. Reduced human movement during COVID-19 offered a rare chance to study how seasonal flu is affected during a pandemic.

we are finally getting a deeper insight into the global distribution patterns of seasonal flu and other respiratory viruses

During the COVID-19 pandemic seasonal flu levels dropped worldwide due to restrictions on movement and mixing. However, once air travel returned there was a rapid bounce back, showing that the virus was in most cases maintained during the pandemic with continued viral movements and accumulation of genetic diversity.

Lead author of the study, Zhiyuan Chen said, ‘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.'

Tropical climates, like those found across South and East Asia, allow for continued flu transmission year-round, thereby leading to a greater variety of strains. Increased capacity for virus tracking during the COVID-19 pandemic has also highlighted the role of regions like Africa and West Asia in the virus’s global spread of flu. These areas showed ongoing transmission, partly because they faced fewer COVID-19 restrictions due to lower transmission rates.

Moritz Kraemer, Professor of Epidemiology and Data Science and a director of the Pandemic Genomics programme, said, ‘Increased genomic surveillance capacity established during the COVID-19 pandemic meant that we are finally getting a deeper insight into the global distribution patterns of seasonal flu and other respiratory viruses. These novel and large openly accessible datasets provide an opportunity to learn about the intricate relationships of climate, co-circulating viruses, and human behaviour.’

Further, with this increased global capacity for surveillance of viruses it might be possible to better monitor seasonal flu to reduce the risk of vaccine mismatches, help inform more effective interventions and reduce the virus’s burden on our healthcare systems. This is especially relevant as more regions become suitable for year-round circulation of flu with changes in climatic conditions.

This research was conducted in partnership with the Oxford Martin Programme on Pandemic Genomics, Department of Biology, Pandemic Sciences Institute, Fudan University, and KU Leuven.