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Volume 31, Number 4—April 2025
Research

Antiviral Susceptibility of Influenza A(H5N1) Clade 2.3.2.1c and 2.3.4.4b Viruses from Humans, 2023–2024

Philippe Noriel Q. Pascua1, Anton Chesnokov1, Ha T. Nguyen, Han Di, Juan De La Cruz, Yunho Jang, Andrei A. Ivashchenko, Alexandre V. Ivachtchenko, Erik A. Karlsson, Borann Sar, Chin Savuth, Timothy M. Uyeki, Charles Todd Davis, and Larisa V. GubarevaComments to Author 
Author affiliation: Centers for Disease Control and Prevention, Atlanta, Georgia, USA (P.N.Q. Pascua, A. Chesnokov, H.T. Nguyen, H. Di, J.D.L. Cruz, Y. Jang, T.M. Uyeki, C.T. Davis, L.V. Gubareva); ChemDiv, San Diego, California, USA (A.A. Ivashchenko, A.V. Ivachtchenko); Avisa LLC, Hallandale Beach, Florida, USA (A.V. Ivachtchenko); Virology Unit, Institut Pasteur du Cambodge, Phnom Penh, Cambodia (E.A. Karlsson); US Centers for Disease Control and Prevention Country Office, Phnom Penh (B. Sar); National Institute of Public Health, Ministry of Health, Phnom Penh (C. Savuth)

Main Article

Table 1

M2 blocker susceptibility of highly pathogenic avian influenza A(H5N1) viruses isolated from humans in virus yield reduction assay, 2023–2024*

Influenza A(H5N1) virus 21 hpi
 16h hpi
Virus titer† log reduction in titer‡
 Phenotype§ Virus titer† log reduction in titer
 Phenotype§
Aman Riman Aman Riman
Clade 2.3.2.1c
A/Cambodia/KSH230332/2023 9.9 1.3 1.8 Sensitive 7.6 2.0 2.2 Sensitive
A/Cambodia/NPH230032/2023, M2-S31N
 8.8
 0.2
 0.0
 Resistant
8.5
 0.6
 0.7
 Resistant
Clade 2.3.4.4b
A/Chile/25945/2023 10.1 0.5 0.7 Resistant 8.6 1.2 1.6 Sensitive
A/Texas/37/2024 10.1 1.1 1.7 Sensitive 8.4 3.6 5.5 Sensitive
A/Michigan/90/2024
 NT
 NT
 NT
 NT
7.0
 5.1
 5.1
 Sensitive
Control viruses
A/bald eagle/FL/2022 (H5N1)¶ 9.2 2.2 2.3 Sensitive 7.0 3.4 3.5 Sensitive
A/Wisconsin/53/2009 (H1N1)pdm09# 7.6 3.0 3.1 Sensitive 6.6 3.0 3.7 Sensitive
A/California/07/2009 (H1N1)pdm09, M2-S31N# 5.8 0.0 0.2 Resistant 4.8 0.2 0.5 Resistant

*Italics indicate differences in phenotypes. Aman, amantadine; hpi, hours postinfection; NT, not tested; riman, rimantadine; TCID50, 50% tissue-culture infectious dose. †log10 TCID50/mL. ‡Maximum reduction in titer observed at M2 blocker concentrations <1 µg/mL. §Virus yields were determined in MDCK-SIAT1 cell culture supernatants collected at 21 or 16 hpi. Data shown are the average of 4 replicates from 2 independent experiments. Resistance to M2 blockers is defined as <1 log10 reduction in infectious virus yield at drug concentration <1 µg/mL. Concentration range of the M2 blockers used was 100–1,600 ng/mL. ¶Clade 2.3.4.4b influenza A(H5N1) virus, A/bald eagle/Florida/W22–134-OP/2022 (GISAID identification no. EPI_ISL_15063846; https://www.gisaid.org), was used as a control virus. #Control seasonal influenza A(H1N1)pdm09 viruses with or without the resistance marker M2-S31N.

Main Article

1These first authors contributed equally to this article.

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Page updated: March 07, 2025
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