A comprehensive study has unveiled significant disruptions to the circulation and evolution of seasonal influenza viruses during the pandemics of 2009 A/H1N1 and COVID-19, particularly within Southeastern Asia. This region has long been recognized as pivotal to the global migration network of seasonal influenza, and researchers have now highlighted how these recent pandemics have shifted influenza dynamics across several years.
Seasonal influenza remains a global public health concern, with millions infected each year. The findings from this study, which utilized genetic, epidemiological, and airline travel data from 2007 to 2023, show stark differences between how influenza A/H3N2 and B/Victoria were affected by the two pandemics. Researchers noted, "We find higher persistence of A/H3N2 than B/Victoria circulation in Southeastern Asia, which is pivotal for global influenza dynamics." During the COVID-19 pandemic, the typical seasonal waves of influenza migration from Southeastern Asia to temperate regions ceased almost entirely for both subtypes, substantially altering patterns observed during previous years.
Historically, Southeastern Asia has been recognized as a fertile ground for the emergence and transmission of various influenza strains. Seasonal patterns often follow cyclical trends, with distinct peaks during colder months. The study aimed to investigate how external shocks, such as pandemics, influenced these typical circulation patterns. Researchers established periods during which pandemic influences were assessed—the 2009 A/H1N1 pandemic spanned from July 2009 to June 2010, whereas the COVID-19 pandemic was defined from July 2020 to June 2021. Both periods were juxtaposed with interpandemic seasons to ascertain changes.
Data analysis revealed considerable variations during the COVID-19 pandemic, where traditional autumn-winter movement waves of influenza viruses from Southeastern Asia dissipated almost completely. One of the pivotal factors was the introduction of strict non-pharmaceutical interventions (NPIs), such as travel restrictions, which led to decreased human mobility. The study's findings suggested, "The COVID-19 pandemic led to considerable disruptions, with autumn-winter movement waves from Southeastern Asia to temperate regions interrupted for both subtypes." Whereas, during the A/H1N1 pandemic, the disruption was primarily noted for the A/H3N2 subtype, leaving B/Victoria movement patterns somewhat intact due to less stringent measures at the time.
The researchers illustrated how A/H3N2 exhibited significant changes during both pandemics, with considerable impacts on its antigenic evolution. While antigenic shifts were noted to follow predictable patterns prior to these pandemics, the onset of the COVID-19 pandemic resulted in delays of nearly five months for new strains to emerge. During the A/H1N1 pandemic, such shifts occurred much earlier—approximately ten months in advance—indicating the unique immunological pressures exerted by the introduction of the new virus.
Interestingly, the study also uncovered notable distinctions within the internal movement networks of influenza viruses. The internal circulation structure among Southeastern Asian regions altered dramatically as well. During the COVID-19 pandemic, there was up to 85% reduction in internal viral movements compared to interpandemic years. On the contrary, the A/H1N1 pandemic resulted in around 52% decline during its peak seasons. Researchers noted this divergence suggests, "Our findings highlight how the A/H3N2 internal migration network within Southeastern Asia is characterized by dynamic change over time as human mobility patterns fluctuate."
These disruptions prompt significant public health concerns. First, they stress the necessity for maintaining rigorous surveillance systems across Southeastern Asia, enhancing the adaptability of responses as new viral strains emerge. The study suggests, "Understanding the circulation patterns during these two pandemics may boost preparedness against influenza outbreaks during interpandemic seasons and future public health emergencies."
The research holds several important lessons for the future management of seasonal influenza, as the observed effects during the COVID-19 pandemic could indicate lasting changes to seasonal patterns. Vaccination strategies will need to remain flexible to accommodate the shifts examined, focusing not just on immediate immunological responses but also on long-term forecasting of evolutionary dynamics and potential future viral behaviors.
Overall, the study provides key insights on the interplay between human mobility, public health interventions, and viral evolution, underlining the need to prepare for recurrent disruptions faced by health systems worldwide due to pandemics.