Spread of Seasonal Flu Returns to Pre-Pandemic Levels After End of COVID-19 Global Emergency

New data show how the spread of seasonal flu viruses evolved due to the COVID-19 pandemic. Investigators emphasized that increased global capacity for surveillance could help monitor seasonal influenza to reduce the risk of vaccine mismatch, inform effective interventions, and reduce the burden on health care systems.¹

Influenza, Flu, Transmission, COVID-19 Pandemic | Image Credit: © Dr_Microbe | stock.adobe.com

"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 behavior,” Moritz Kraemer, professor in the department of biology and pandemic sciences institute at the University of Oxford, said in a news release.¹

According to the study authors, seasonal influenza is strongly associated with global air travel and the evolution of distinct viruses that are not covered by vaccines and/or infection-induced immune responses. Due to social distancing and the stay-at-home guidelines during the COVID-19 pandemic, a decline in flu incidence was seen, which caused changes in population immunity and virus diversity. Although there has been a resurgence in transmission and dispersal, the virus B/Yamagata has been rarely seen since the beginning of the pandemic in 2020, according to the investigators.²

The study authors identified 3 periods of the pandemic: the pre-pandemic period (January 2017 to March 2020), the pandemic period (April 2020 to April 2023), and post-pandemic period (May 2023 to March 2024). The pandemic period was divided into the acute phase (April 2020 to March 2021) and the transition phase (April 2021 to April 2023). They used data from the Global Influenza Surveillance and Response System and data from national surveillance databases.²

In the acute phase, investigators found that influenza testing was stable but doubled in the transition phase. The positivity rates decreased by 99.2% for H1N1pdm09, 99.2% for H3N2, 96.9% for B/Victoria, and about 100% for B/Yamagata during the acute phase compared with the pre-pandemic phase. Furthermore, after the acute phase, investigators found an H3N2 epidemic in the Northern Hemisphere during winter 2021 and 202 as well as outbreaks of H1N1pdm09, H3N2, and B/Victoria infections in the Northern Hemisphere winter 2022 to 2023. The transmission of the 3 viruses either resumed or continued in the post-pandemic period with the exception of B/Yamagata, which had about 20 cases or less.²

The investigators found that the circulation of influenza continued in Africa, but it had less of an influence on global patterns, which they estimated was due to less international travel. Additionally, the data showed strong support for the association of international air travel between-region virus movement. When the global emergency ended in 2023, seasonal influenza returned to pre-pandemic patterns and the global lineage dynamics were similar to pre-pandemic.²

“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” Hongjie Yu, professor at the Fudan University, said in the news release.¹

REFERENCES

1. New study traces impact of COVID-19 pandemic on global movement and evolution of seasonal flu. News release. University of Oxford. November 7, 2024. Accessed November 18, 2024. https://www.sciencedaily.com/releases/2024/11/241107160545.htm

2. Chen Z, Tsui JL, Gutierrez B, et al. COVID-19 pandemic interventions reshaped the global dispersal of seasonal influenza viruses. Science. 2024;386(6722):eadq3003. doi:10.1126/science.adq3003