Influenza virus strains circulating 2024
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Circulating Influenza Virus Strains in 2024: Global and Regional Overview
Dominant Influenza Subtypes and Lineages in 2023–2024
During the 2023/24 influenza season, the most commonly circulating influenza viruses worldwide were A(H1N1)pdm09, A(H3N2), and B/Victoria lineage viruses. Across Europe, including the EU/EEA, A(H1N1)pdm09 made up about 75% of subtyped influenza A viruses, while A(H3N2) accounted for 25%. All characterized influenza B viruses belonged to the B/Victoria lineage, with no B/Yamagata detected. The dominant genetic clades were 5a.2a for A(H1N1)pdm09, 2a.3a.1 for A(H3N2), and V1A.3a.2 for B/Victoria-lineage viruses 136.
Similar patterns were observed in regional studies. In Shandong Province, China, A(H3N2) was the dominant strain, while A(H1N1)pdm09 and B(Victoria) also circulated. In Bulgaria, A(H1N1)pdm09 was the most prevalent, followed by A(H3N2) and B/Victoria 26. In Linyi, northern China, both A(H3N2) and A(H1N1)pdm09 were detected, with A(H3N2) causing major epidemic waves .
Genetic Evolution and Antigenic Drift
Genetic diversification of influenza viruses continued in 2024. The A(H3N2) viruses, especially those in clade 2a.3a.1, showed significant antigenic drift, with some strains being antigenically distinct from the 2023/24 vaccine virus. This drift led to reduced vaccine efficacy in some regions, such as Romania and Shandong, where A(H3N2) strains had a poor match with the current vaccine but were closer to the 2024/25 vaccine strain 1234+2 MORE.
A(H1N1)pdm09 viruses generally remained antigenically similar to the vaccine strain, but some genetic drift was observed, particularly in Sicily, where about 40% of sequences showed partial drift from the recommended vaccine strain 1367. B/Victoria viruses also continued to evolve, but all detected strains were closely related to the vaccine component 16.
Vaccine Effectiveness and Recommendations
The antigenic drift in A(H3N2) viruses led to updates in the World Health Organization’s vaccine recommendations for the 2024/25 northern hemisphere season, introducing a new A(H3N2) component while retaining the current A(H1N1)pdm09 and B/Victoria components 123. Studies from multiple regions highlighted the importance of ongoing surveillance and timely vaccine updates to maintain effectiveness 1236+1 MORE.
Antiviral Resistance and Pathogenicity
Most circulating influenza viruses in 2024 remained sensitive to neuraminidase inhibitors and baloxavir marboxil. However, isolated cases of oseltamivir resistance were reported, such as an A(H1N1)pdm09 strain with the H275Y mutation in Shandong . No increased pathogenicity or virulence markers were found in A(H3N2) viruses from lethal cases in Russia, and no widespread drug resistance was detected 58.
Zoonotic Influenza and Avian Strains
Globally, 2024 saw continued outbreaks of highly pathogenic avian influenza, particularly A(H5N1), affecting poultry, wild birds, and mammals, with some human infections reported. The A(H5N1) viruses detected in Russia were genetically similar to vaccine strains and did not show increased pathogenicity or drug resistance. Other avian subtypes, such as A(H5N6), A(H5N8), and A(H7), also circulated in various regions .
Conclusion
In 2024, influenza virus circulation was marked by the dominance of A(H1N1)pdm09, A(H3N2), and B/Victoria lineages, with ongoing genetic evolution and antigenic drift, especially in A(H3N2) viruses. These changes impacted vaccine effectiveness and prompted updates to vaccine recommendations. While most viruses remained sensitive to antivirals, isolated resistance cases highlight the need for continued surveillance. The global situation with avian influenza remains a concern, emphasizing the importance of monitoring both seasonal and zoonotic influenza viruses.
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