COVID-19 restrictions altered global dispersal of influenza viruses
Summary author: Walter Beckwith
Although travel restrictions and social measures during the COVID-19 pandemic led to a dramatic global drop in seasonal influenza cases, certain influenza lineages in specific regions kept the virus circulating and evolving, according to a new study. This was true in tropical areas with fewer travel restrictions, for example, including South and West Asia. The spread of seasonal influenza is closely tied to social behavior, particularly air travel, and to the periodic evolution of new virus strains that evade immunity from prior infections or vaccinations. In 2020, nonpharmaceutical interventions (NPIs) introduced to combat COVID-19 – such as lockdown policies, mandated social distancing, masking, and travel bans – dramatically impacted influenza virus transmission and evolution. Due to these interventions, cases of seasonal influenza caused by A subtypes H1N1 and H3N2, as well as influenza B subtypes Victoria and Yamagata, declined sharply across the globe.
Here, Zhiyuan Chen and colleagues investigated how these changes affected the spread, distribution, and evolutionary dynamics of seasonal influenza lineages. Using a phylodynamic approach, Chen et al. 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.
Journal
Science
Article Title
COVID-19 pandemic interventions reshaped the global dispersal of seasonal influenza viruses
Article Publication Date
8-Nov-2024
New study traces impact of COVID-19 pandemic on global movement and evolution of seasonal flu
University of Oxford
- Increased capabilities for genomic surveillance have offered new insights into global viral evolution;
- Seasonal flu showed a ‘remarkable’ bounce back to pre-pandemic levels once international air travel resumed;
- Regions with fewer COVID-19 restrictions were associated with sustained flu virus transmission.
Seasonal influenza epidemics impose substantial burdens on healthcare systems and cause >5 million hospitalizations of adults each year. The current approach to influenza vaccine development requires comprehensive surveillance of circulating strains, which are constantly moving from continent to continent. The reduction in global human mobility during the COVID-19 pandemic provided a unique opportunity to evaluate how seasonal influenza is impacted during pandemics.
In this new study, an international team of researchers, including from the University of Oxford, Fudan University, and KU Leuven, combined data on the spread of seasonal influenza, its genetic makeup, and international travel patterns to study how seasonal flu 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.
During the COVID-19 pandemic, seasonal influenza levels dropped worldwide due to COVID-19 pandemic-related restrictions on population movement and mixing. However, the subsequent rapid bounce back of influenza once air travel returned to pre-pandemic levels showed 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 (University of Oxford and Fudan University) says “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 creating a broader range of flu strains and increasing overall viral diversity. The increased capacity for virus genomic surveillance during the COVID-19 pandemic has provided more detail on the role of other regions, such as Africa and West Asia, in the global circulation of influenza. These regions also showed evidence of sustained transmission and during the pandemic had relatively less restrictions on movement, in part due to lower levels of COVID-19 transmission.
Co-author Professor Moritz Kraemer (Department of Biology and Pandemic Sciences Institute, University of Oxford) says: “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. 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 influenza to reduce the risk of vaccine mismatches, help inform more effective interventions, and reduce the burden of seasonal influenza on our healthcare systems. This is especially relevant as more regions become suitable for year-round circulation of influenza with changes in climatic conditions.
Co-author Professor Hongjie Yu from Fudan University says: “Further efforts should still focus on the continuing surveillance of seasonal influenza viruses and other respiratory pathogens, particularly resource-limited regions. The established surveillance systems for seasonal respiratory pathogens could also play an extremely vital role when the next pandemic emerges in the future.”
Notes to editors
The paper ‘COVID-19 pandemic interventions reshaped the global dispersal of seasonal influenza viruses’ will be published in the journal Science at https://doi.org/10.1126/science.adq3003. Advance copies of the paper may be obtained from the Science press package, SciPak, at https://www.eurekalert.org/press/scipak/ or by contacting scipak@aaas.org.
Interviews with the corresponding author are available on request.
Media contact: Professor Moritz Kraemer: moritz.kraemer@biology.ox.ac.uk
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Reconstruction of historical seasonal influenza patterns and individual lifetime infection histories in humans based on antibody profiles
PLOS
image:
Chinese lanterns released over a town near Guangzhou, China (one of the study locations).
view moreCredit: Jon Read (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/)
In your coverage, please use this URL to provide access to the freely available paper in PLOS Biology: http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.3002864
Article Title: Reconstructed influenza A/H3N2 infection histories reveal variation in incidence and antibody dynamics over the life course
Author Countries: United Kingdom, China, United States
Funding: see manuscript
Journal
PLOS Biology
Method of Research
Observational study
Subject of Research
People
COI Statement
Competing interests: I have read the journal’s policy and the authors of this manuscript have the following competing interests: DATC declares research funding from Merck, Sharp and Dohme and from Pfizer for research unrelated to this manuscript. All other authors have declared that no competing interests exist.
Journal
Science
Article Title
COVID-19 pandemic interventions reshaped the global dispersal of seasonal influenza viruses
