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Volume 31, Number 5—May 2025

Dispatch

Influenza A(H1N1)pdm09 Virus with Reduced Susceptibility to Baloxavir, Japan, 2024

Emi TakashitaComments to Author , Hiroko Morita, Shiho Nagata, Seiichiro Fujisaki, Hideka Miura, Tatsuya Ikeda, Kenichi Komabayashi, Mika Sasaki, Yohei Matoba, Tomoko Takahashi, Naomi Ogawa, Katsumi Mizuta, Sueshi Ito, Noriko Kishida, Kazuya Nakamura, Masayuki Shirakura, Shinji Watanabe, and Hideki Hasegawa
Author affiliation: Author affiliations: National Institute of Infectious Diseases, Tokyo, Japan (E. Takashita, H. Morita, S. Nagata, S. Fujisaki, H. Miura, N. Kishida, K. Nakamura, M. Shirakura, S. Watanabe, H. Hasegawa); Yamagata Prefectural Institute of Public Health, Yamagata, Japan (T. Ikeda, K. Komabayashi, M. Sasaki, Y. Matoba, T. Takahashi, N. Ogawa, K. Mizuta); Ito Clinic, Yamagata (S. Ito)

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Table 1

Antiviral drug susceptibilities of influenza A(H1N1)pdm09 viruses, Japan, 2024*

Influenza virus GISAID isolate ID PA substitution Specimen collection Baloxavir† (fold-change) NA inhibitors‡ (fold-change)
Oseltamivir Peramivir Zanamivir Laninamivir
A/Yamagata/103/2024 19183931 I38N 2024 Mar 12 388.20 (90) 0.21 (0.9) 0.11 (1.4) 0.43 (1.1) 0.52 (0.9)
A/Yamagata/333/2023 19045749 I38T 2023 Dec 15 929.18 (216) 0.26 (1.1) 0.08 (1.0) 0.32 (0.8) 0.44 (0.8)
A/Yamagata/122/2023 18744526 None 2023 Sep 21 4.05 (0.9) 0.23 (1.0) 0.08 (1.0) 0.46 (1.2) 0.70 (1.2)
A/Yamagata/127/2023 18744528 None 2023 Oct 2 5.38 (1.3) 0.22 (1.0) 0.08 (1.0) 0.44 (1.2) 0.70 (1.2)
A/Yamagata/135/2023 18799184 None 2023 Oct 15 6.08 (1.4) 0.24 (1.0) 0.08 (1.0) 0.43 (1.1) 0.71 (1.3)
A/Yamagata/177/2023 18853702 None 2023 Nov 4 6.69 (1.6) 0.22 (1.0) 0.06 (0.8) 0.25 (0.7) 0.36 (0.6)
A/Yamagata/292/2023 18987233 None 2023 Dec 4 5.15 (1.2) 0.22 (1.0) 0.09 (1.1) 0.35 (0.9) 0.52 (0.9)
A/Yamagata/312/2023 18987234 None 2023 Dec 11 5.05 (1.2) 0.22 (1.0) 0.11 (1.4) 0.38 (1.0) 0.49 (0.9)
A/Yamagata/336/2023 19045750 None 2023 Dec 15 6.47 (1.5) 0.23 (1.0) 0.09 (1.1) 0.30 (0.8) 0.42 (0.7)
A/Yamagata/104/2024 19201115 None 2024 Mar 27 4.32 (1.0) 0.28 (1.2) 0.09 (1.1) 0.33 (0.9) 0.50 (0.9)

*Influenza A(H1N1)pdm09 for 2023–24 was 4.30 + 2.38 (n = 214) for baloxavir and 0.23 + 0.06 (n = 208) for oseltamivir, 0.08 + 0.02 (n = 208) for peramivir, 0.38 + 0.12 (n = 208) for zanamivir, and 0.56 + 0.14 (n = 208) for laninamivir. EC50, 50% effective concentration; fold-change, fold-change in EC50 and IC50 values compared with the median values of 2023–24 seasonal pH1N1 viruses isolated in Japan; GISAID, GISAID EpiFlu database (http://www.gisaid.org); IC50, 50% inhibitory concentration; NA, neuraminidase; PA, polymerase acidic protein. †Mean EC50, nmol/L. Mean EC50 values of triplicate reactions in a single run were determined by using a focus reduction assay. ‡Mean IC50, nmol/L. Mean IC50 values of duplicate reactions in a single run were determined by using a fluorescence-based NA inhibition assay.

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