During the COVID-19 pandemic, global influenza cases dropped dramatically due to lockdowns and travel restrictions. However, in regions with fewer restrictions, influenza continued to circulate, fueling its evolution.
Research shows that global air travel is key to influenza spread, and while many lineages returned post-pandemic, influenza B/Yamagata may have gone extinct.
During the COVID-19 pandemic, travel restrictions and social measures led to a sharp worldwide decrease in seasonal flu cases. However, certain influenza strains continued to circulate and evolve in regions with fewer restrictions, particularly in tropical areas like South and West Asia. The spread of seasonal flu is strongly influenced by social behaviors, especially air travel, and by the periodic emergence of new strains that evade immunity from previous infections or vaccinations.
In 2020, nonpharmaceutical interventions (NPIs) aimed at controlling COVID-19—such as lockdowns, mandated social distancing, masking, and travel bans—significantly disrupted influenza transmission and evolution. As a result, global cases of seasonal flu, including A subtypes H1N1 and H3N2 and B subtypes Victoria and Yamagata, saw a dramatic decline.
In this study, Zhiyuan Chen and colleagues investigated how these changes affected the spread, distribution, and evolutionary dynamics of seasonal influenza lineages. Using a phylodynamic approach, the researchers combined epidemiological, genetic, and international travel data from before, during, and after the COVID-19 pandemic and found that the onset of the pandemic led to a shift in the intensity and structure of international influenza transmission.
Although influenza cases significantly dropped globally during the pandemic’s peak, in South Asia and West Asia, regions that had relatively fewer pandemic restrictions, the circulation of influenza A and influenza B/Victoria lineages, respectively, continued. That circulation served as important evolutionary sources, or “phylogenetic trunk locations,” of influenza viruses during the pandemic period.
By March 2023, as global air traffic resumed, the circulation of influenza lineages returned to pre-pandemic levels, highlighting the virus’ resilience to long-term disruption and its reliance on global air travel patterns to spread. Notably, however, the findings also show that the influenza B/Yamagata lineage appears to have disappeared since the start of the pandemic, suggesting that the lineage may have since gone extinct.
“The study by Chen et al. further reinforces 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,” write Pejman Rohani and Justin Bahl in a related Perspective.
Reference: “COVID-19 pandemic interventions reshaped the global dispersal of seasonal influenza viruses” by Zhiyuan Chen, Joseph L.-H. Tsui, Bernardo Gutierrez, Simon Busch Moreno, Louis du Plessis, Xiaowei Deng, Jun Cai, Sumali Bajaj, Marc A. Suchard, Oliver G. Pybus, Philippe Lemey, Moritz U. G. Kraemer and Hongjie Yu, 8 November 2024, Science.
DOI: 10.1126/science.adq3003
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