![\"\"](\"http:\/\/www.med.niigata-u.ac.jp\/pub\/wppublichealth\/wp-content\/uploads\/2026\/02\/\u30b9\u30e9\u30a4\u30c91.jpg\")
<\/a> ![\"\"](\"http:\/\/www.med.niigata-u.ac.jp\/pub\/wppublichealth\/wp-content\/uploads\/2026\/02\/\u30b9\u30e9\u30a4\u30c92.jpg\")
<\/a> ![\"\"](\"http:\/\/www.med.niigata-u.ac.jp\/pub\/wppublichealth\/wp-content\/uploads\/2026\/02\/\u30b9\u30e9\u30a4\u30c93.jpg\")
<\/a> ![\"\"](\"http:\/\/www.med.niigata-u.ac.jp\/pub\/wppublichealth\/wp-content\/uploads\/2026\/02\/\u30b9\u30e9\u30a4\u30c94.jpg\")
<\/a><\/p>\n\u3010\u521d\u8a3a\u6642\u691c\u4f53\u3011
\n\u521d\u8a3a\u6642\u306b\u63a1\u53d6\u3055\u308c\u305f173\u4ef6\u306e\u691c\u4f53\u306e\u5185\u8a33\u306f\u3001\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6\u30a6\u30a4\u30eb\u30b9A(H1N1)pdm09\u304c0\u4ef6\uff080%\uff09\u3001\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6\u30a6\u30a4\u30eb\u30b9A(H3N2)\u304c158\u4ef6\uff0891.3%\uff09\u3001\u4e9c\u578b\u5224\u5b9a\u4e0d\u80fd\u306eA\u578b\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6\u30a6\u30a4\u30eb\u30b9\u306f2\u4ef6\uff081.2\uff05\uff09\u3001\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6\u30a6\u30a4\u30eb\u30b9B\/Victoria\u304c7\u4ef6\uff084.0%\uff09\u3001\u7cfb\u7d71\u304c\u5224\u5b9a\u3067\u304d\u306a\u304b\u3063\u305fB\u578b\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6\u304c0\u4ef6\uff080\uff05\uff09\u3067\u3057\u305f\u3002\u307e\u305f\u3001A\u578b\u304a\u3088\u3073B\u578b\u306e\u6df7\u5408\u611f\u67d3\u306f0\u4ef6\uff080.0\uff05\uff09\u3001RT-PCR\u9670\u6027\u306e\u691c\u4f53\u306f6\u4ef6\uff083.5%\uff09\u3067\u3057\u305f\uff08\u88681\uff09\u3002<\/p>\n
\u6cbb\u7642\u524d\u306b\u63a1\u53d6\u3055\u308c\u305f\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6A\u578b\u306e\u691c\u4f53\u306b\u3064\u3044\u3066\u3001\u30b5\u30a4\u30af\u30ea\u30f3\u30b0\u30d7\u30ed\u30fc\u30d6RT-PCR\u6cd5\u306b\u3088\u308aPA\/I38T\u5909\u7570\u306e\u691c\u51fa\u3092\u8a66\u307f\u305f\u7d50\u679c\u3001158\u4ef6\u306eA(H3N2)\u306e\u3046\u3061PA\/I38T\u5909\u7570\u967d\u6027\u306e\u691c\u4f53\u304c2\u691c\u4f53\u78ba\u8a8d\u3055\u308c\u307e\u3057\u305f\uff08\u691c\u51fa\u7387\uff1a1.3\uff05\uff09\uff08\u88682-1\uff09\u3002<\/p>\n
\u6b21\u4e16\u4ee3\u30b7\u30fc\u30af\u30a8\u30f3\u30b9\u89e3\u6790\u306f\u3001\u521d\u8a3a\u6642\u691c\u4f53158\u4ef6\u306e\u3046\u3061A(H3N2) 65\u4ef6\u3001B\/Victoria 6\u4ef6\u3092\u5bfe\u8c61\u306b\u5b9f\u65bd\u3057\u307e\u3057\u305f\u3002\u305d\u306e\u7d50\u679c\u3001\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u8010\u6027\u306b\u95a2\u9023\u3059\u308bPA\/I38T\u5909\u7570\u3092A(H3N2)\u3067 2\u4ef6\u691c\u51fa\u3057\u307e\u3057\u305f\uff08\u691c\u51fa\u7387\uff1a3.1%\uff09\uff08\u88682-2\uff09\u3002\u3053\u308c\u30892\u691c\u4f53\u306e\u30d0\u30ea\u30a2\u30f3\u30c8\u30b3\u30fc\u30eb\u89e3\u6790\u3067\u306f\u3001PA\/I38T\u5909\u7570\u306e\u5272\u5408\u304c98%\u4ee5\u4e0a\u3067\u3042\u308a\u3001\u91ce\u751f\u578b\u306f\u307b\u3068\u3093\u3069\u691c\u51fa\u3055\u308c\u305a\u3001PA\/I38T\u5909\u7570\u30a6\u30a4\u30eb\u30b9\u304c\u307b\u307c\u5168\u3066\u3092\u5360\u3081\u308b\u72b6\u614b\u3067\u3057\u305f\u30021\u4f8b\u306f\u521d\u8a3a\u6642\u691c\u4f53\u306e\u307f\u3067\u3001\u81e8\u5e8a\u7d4c\u904e\u306e\u8a73\u7d30\u306f\u78ba\u8a8d\u3067\u304d\u3066\u3044\u307e\u305b\u3093\u3002\u3082\u30461\u4f8b\u306f\u3001\u6cbb\u7642\u306f\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u3067\u884c\u308f\u308c\u307e\u3057\u305f\u3002\u6709\u71b1\u6642\u9593\u306f4.8\u6642\u9593\u3067\u3057\u305f\u3002\u518d\u8a3a\u691c\u4f53\u3082\u63a1\u53d6\u3055\u308c\u307e\u3057\u305f\u304c\u3001\u30a6\u30a4\u30eb\u30b9\u91cf\u304c\u5c11\u306a\u304f\u89e3\u6790\u3067\u304d\u307e\u305b\u3093\u3067\u3057\u305f\u3002<\/p>\n
\u307e\u305f\u3001\u521d\u8a3a\u6642\u306eA(H3N2)\u691c\u4f531\u4ef6\u3067\u3001NGS\u30d0\u30ea\u30a2\u30f3\u30c8\u30b3\u30fc\u30eb\uff08\u5909\u7570\u691c\u51fa\u95be\u50245%\uff09\u306b\u3088\u308aPA\/E23V\u304c5.7%\u691c\u51fa\u3055\u308c\u307e\u3057\u305f\u3002E23V\u306f\u8010\u6027\u95a2\u9023\u5909\u7570\u30ea\u30b9\u30c8\uff08\u4f8b\uff1aE23G\/K\/R\uff09\u306b\u542b\u307e\u308c\u305a\u6587\u732e\u5831\u544a\u3082\u78ba\u8a8d\u3067\u304d\u306a\u3044\u305f\u3081\u3001\u53c2\u8003\u6240\u898b\u3068\u3057\u3066\u8a18\u8f09\u3057\u307e\u3059\uff08\u88682\u20102\u306b\u306f\u611f\u53d7\u6027\u682a\u3068\u3057\u3066\u8a08\u4e0a\uff09\u3002<\/p>\n
\u30b5\u30f3\u30ac\u30fc\u6cd5\u306b\u3088\u308bPA\u907a\u4f1d\u5b50\u5909\u7570\u306e\u89e3\u6790\u306f\u3001\u73fe\u6642\u70b9\u3067\u306f\u672a\u5b9f\u65bd\u3067\u3059\u3002<\/p>\n
A(H3N2) 65\u4ef6\u3001B\/Victoria 6\u4ef6\u304b\u3089\u3001\u30aa\u30bb\u30eb\u30bf\u30df\u30d3\u30eb\u306b\u5bfe\u3059\u308b\u611f\u53d7\u6027\u4f4e\u4e0b\u306b\u95a2\u9023\u3059\u308bNA\u907a\u4f1d\u5b50\u5909\u7570\u306f\u3001\u6b21\u4e16\u4ee3\u30b7\u30fc\u30af\u30a8\u30f3\u30b9\u6cd5\u3067\u691c\u51fa\u3055\u308c\u307e\u305b\u3093\u3067\u3057\u305f\uff08\u88684\uff09\u3002<\/p>\n
\u3010\u518d\u8a3a\u6642\u691c\u4f53\u3011
\n\u518d\u8a3a\u6642\u691c\u4f53\u306f87\u540d\u304b\u3089\u63a1\u53d6\u3055\u308c\u3001\u3053\u306e\u3046\u3061\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u6cbb\u7642\u3092\u53d7\u3051\u305f\u60a3\u800549\u540d\uff08A(H3N2) 48\u540d\u3001B\/Victoria 1\u540d\uff09\u306b\u3064\u3044\u3066\u3001\u6cbb\u7642\u5f8c3\uff5e7\u65e5\u76ee\u306b\u518d\u8a3a\u6642\u306e\u4e0a\u6c17\u9053\u691c\u4f53\u3092\u63d0\u4f9b\u3044\u305f\u3060\u304d\u307e\u3057\u305f\uff08\u88681\uff09\u3002<\/p>\n
\u3053\u308c\u3089\u306e\u691c\u4f53\u306b\u5bfe\u3057\u3066\u3001\u30b5\u30a4\u30af\u30ea\u30f3\u30b0\u30d7\u30ed\u30fc\u30d6RT-PCR\u6cd5\u3092\u7528\u3044\u3066PA\/I38T\u5909\u7570\u306e\u6709\u7121\u3092\u8abf\u3079\u305f\u7d50\u679c\u3001A(H3N2) 48\u691c\u4f53\u4e2d11\u691c\u4f53\u3067PA\/I38T\u5909\u7570\u304c\u691c\u51fa\u3055\u308c\u307e\u3057\u305f\uff08\u691c\u51fa\u7387\uff1a22.9\uff05\uff09\u3002\u3053\u306e\u3046\u30611\u691c\u4f53\u306f\u3001PA\/I38T\u5909\u7570\u306e\u3042\u308b\u30a6\u30a4\u30eb\u30b9\u306e\u307f\u304c\u691c\u51fa\u3055\u308c\u3001\u4ed610\u691c\u4f53\u306f\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u611f\u53d7\u6027\u682a\u3068\u8010\u6027\u5909\u7570\u682a\u304c\u6df7\u5728\u3057\u3066\u3044\u308b\u3068\u8003\u3048\u3089\u308c\u307e\u3057\u305f\u3002\u6df7\u5728\u4f8b\u306e\u3046\u30611\u4f8b\u306f\u3001\u518d\u8a3a\u306e1\u65e5\u5f8c\u306b\u3082\u3046\u4e00\u56de\u691c\u4f53\u304c\u63a1\u53d6\u3055\u308c\u3066\u304a\u308a\u3001\u305d\u306e\u691c\u4f53\u304b\u3089\u306fPA\/I38T\u5909\u7570\u682a\u306e\u307f\u691c\u51fa\u3055\u308c\u307e\u3057\u305f\u3002
\n\u307e\u305f\u3001\u518d\u8a3a\u6642\u306bRT-PCR\u3067\u30a4\u30f3\u30d5\u30eb\u30a8\u30f3\u30b6A(H3N2)\u304c\u967d\u6027\u3067\u3042\u3063\u305f22\u691c\u4f53\u3092\u5206\u6bcd\u3068\u3057\u305f\u5834\u5408\u3001PA\/I38T\u5909\u7570\u306f11\u691c\u4f53\u3067\u691c\u51fa\u3055\u308c\u3001\u691c\u51fa\u5272\u5408\u306f50.0\uff05\uff0811\/22\uff09\u3067\u3057\u305f\uff08\u88682-1\uff09\u3002<\/p>\n
87\u4ef6\u306e\u518d\u8a3a\u6642\u691c\u4f53\u306e\u3046\u3061\u3001NGS\u691c\u67fb\u3092\u5b9f\u65bd\u3057\u305f\u306e\u306f30\u4ef6\u3067\u3001\u305d\u306e\u3046\u306122\u4ef6\u304c\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u6295\u4e0e\u5f8c\u30018\u4ef6\u304c\u30aa\u30bb\u30eb\u30bf\u30df\u30d3\u30eb\u6295\u4e0e\u5f8c\u306e\u691c\u4f53\u3067\u3057\u305f\uff08\u88682-2\u3001\u88684\uff09\u3002\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u6295\u4e0e\u5f8c\u306e22\u691c\u4f53\uff08\u5168\u3066A(H3N2)\uff09\u306eNGS\u89e3\u6790\u3092\u5b9f\u65bd\u3057\u307e\u3057\u305f\u300215\u4ef6\u3067\u30c7\u30fc\u30bf\u89e3\u6790\u304c\u53ef\u80fd\u3067\u3001PA\u907a\u4f1d\u5b50\u306e\u5909\u7570\u306f11\u4ef6\u3067\u78ba\u8a8d\u3055\u308c\u307e\u3057\u305f\uff08\u691c\u51fa\u7387\uff1a73.3\uff05\uff09\u3002PA\u5909\u7570\u306e\u691c\u51fa\u7387\u306f\u89e3\u6790\u657022\u4ef6\u3092\u5206\u6bcd\u3068\u3059\u308b\u306850.0\uff05\uff0811\/22\uff09\u3001\u89e3\u6790\u304c\u6210\u529f\u3057\u305f15\u4ef6\u3092\u5206\u6bcd\u3068\u3059\u308b\u306873.3\uff05\uff0811\/15\uff09\u3067\u3057\u305f\u3002\u78ba\u8a8d\u3055\u308c\u305f\u5909\u7570\u306e\u5185\u8a33\u306f\u3001PA\/I38T\u304c7\u4ef6\uff08\u30d0\u30ea\u30a2\u30f3\u30c8\u30b3\u30fc\u30eb\uff1a7.4\uff5e54.7\uff05\uff09\u3001PA\/I38M\u304c3\u4ef6\uff08\u30d0\u30ea\u30a2\u30f3\u30c8\u30b3\u30fc\u30eb\uff1a6.8\uff5e35.4\uff05\uff09\u3001PA\/I38T\u3068PA\/I38M\u304c\u540c\u6642\u306b\u691c\u51fa\u3055\u308c\u305f\u691c\u4f53\u304c1\u4ef6\uff08PA\/I38T\uff1a10.4\uff05\u3001PA\/I38M\uff1a14.4\uff05\uff09\u3067\u3057\u305f\u3002\u3053\u308c\u3089\u306e11\u4f8b\u306ePA\u5909\u7570\u3092\u6709\u3059\u308bA(H3N2)\u75c7\u4f8b\u306b\u3064\u3044\u3066\u3001\u30d0\u30ed\u30ad\u30b5\u30d3\u30eb\u6295\u4e0e\u5f8c\u306e\u6709\u71b1\u6642\u9593\u306e\u4e2d\u592e\u5024\u306f\u300133\u6642\u9593\u3067\u3057\u305f\uff08\u7bc4\u56f2\uff1a0-128\u6642\u9593\uff09\u3002<\/p>\n
\u73fe\u5728\u3001\u30b5\u30a4\u30af\u30ea\u30f3\u30b0\u30d7\u30ed\u30fc\u30d6RT-PCR\u6cd5\u3067PA\/I38T\u5909\u7570\u304c\u8a8d\u3081\u3089\u308c\u305f\u691c\u4f53\u3092\u512a\u5148\u7684\u306bNGS\u3067\u89e3\u6790\u3057\u3066\u3044\u308b\u305f\u3081\u3001PA\u5909\u7570\u306e\u691c\u51fa\u983b\u5ea6\u306f\u5b9f\u969b\u3088\u308a\u9ad8\u3044\u53ef\u80fd\u6027\u304c\u3042\u308a\u307e\u3059\u3002<\/p>\n
\u30aa\u30bb\u30eb\u30bf\u30df\u30d3\u30eb\u6295\u4e0e\u5f8c\u306e\u518d\u8a3a\u6642\u306b\u63a1\u53d6\u3055\u308c\u305fA(H3N2)\u691c\u4f538\u4ef6\u306b\u3064\u3044\u3066NGS\u89e3\u6790\u3092\u5b9f\u65bd\u3057\u307e\u3057\u305f\u3002\u89e3\u6790\u53ef\u80fd\u3067\u3042\u3063\u305f1\u4ef6\u304b\u3089\u3001\u30aa\u30bb\u30eb\u30bf\u30df\u30d3\u30eb\u306b\u5bfe\u3059\u308b\u611f\u53d7\u6027\u4f4e\u4e0b\u306b\u95a2\u9023\u3059\u308bNA\u907a\u4f1d\u5b50\u5909\u7570\u306f\u3001\u6b21\u4e16\u4ee3\u30b7\u30fc\u30af\u30a8\u30f3\u30b9\u6cd5\u3067\u691c\u51fa\u3055\u308c\u307e\u305b\u3093\u3067\u3057\u305f\uff08\u88684\uff09\u3002<\/p>\n
Overview of Influenza Circulating Strains and Antiviral Drug-Resistant Strains in Japan During the 2025-2026 Season (As of February 17, 2026)<\/strong> Participating Medical Institutions and Sample Collection<\/strong> Baseline (pre-treatment) specimens<\/strong> For baseline pre-treatment influenza A specimens, we screened for the PA\/I38T substitution using the cycling-probe RT-PCR method. PA\/I38T-positive specimens were identified in 2 of 158 A(H3N2) specimens (detection rate: 1.3%) (Table 2-1).<\/p>\n NGS analysis was performed for 65 A(H3N2) and 6 B\/Victoria specimens among the 158 baseline specimens collected at the initial visit. As a result, the PA\/I38T substitution associated with reduced susceptibility to baloxavir was detected in 2 A(H3N2) specimens (detection rate: 3.1%) (Table 2-2). Variant-calling analysis of these two specimens showed that the PA\/I38T variant frequency was \u226598%, indicating that wild-type virus was barely detectable and that the PA\/I38T variant population accounted for almost the entire viral population in the specimens. One case had only a baseline specimen collected, and detailed clinical course information could not be confirmed. In the other case, baloxavir was used for treatment, and the fever duration was 4.8 hours. A follow-up specimen was collected from this case, but the viral load was low and sequencing analysis was not successful.<\/p>\n In addition, in one baseline A(H3N2) specimen, PA\/E23V was detected at 5.7% by NGS variant calling (variant detection threshold: 5%). Because E23V is not included in the list of baloxavir resistance-associated substitutions by WHO (e.g., E23G\/K\/R) and no published reports have been identified, this finding is described as a reference (this sample is counted as a susceptible virus in Table 2-2).<\/p>\n Sanger sequencing for PA gene substitutions has not been performed to date.<\/p>\n No NA gene substitutions associated with reduced susceptibility to oseltamivir were detected by NGS in the 65 A(H3N2) and 6 B\/Victoria baseline specimens (Table 4).<\/p>\n Follow-up (post-treatment) specimens<\/strong> Using the cycling-probe RT-PCR method, PA\/I38T was detected in 11 of the 48 A(H3N2) follow-up specimens (detection rate: 22.9%). In one specimen, only the PA\/I38T variant virus was detected, whereas in the remaining 10 specimens, baloxavir-susceptible virus and resistance-associated variants were considered to coexist. Among the mixed cases, one patient had an additional specimen collected one day after the follow-up visit, from which only the PA\/I38T variant virus was detected. When analysis was restricted to the 22 follow-up specimens that were RT-PCR positive for influenza A(H3N2), PA\/I38T was detected in 11 specimens, corresponding to a detection proportion of 50.0% (11\/22) (Table 2-1).<\/p>\n Among the 87 follow-up specimens, NGS was performed for 30 specimens, including 22 specimens collected after baloxavir treatment and 8 specimens collected after oseltamivir treatment (Tables 2-2 and 4). NGS analysis was conducted for the 22 post-baloxavir specimens (all A(H3N2)). Sequencing analysis was successful in 15 specimens, and PA gene substitutions were identified in 11 of these specimens (NGS success rate: 73.3%). The detection rate of PA substitutions was 50.0% (11\/22) when calculated using all 22 specimens as the denominator, and 73.3% (11\/15) when calculated using the 15 specimens with successful NGS analysis as the denominator. The detected substitutions consisted of PA\/I38T in 7 specimens (variant call frequencies: 7.4\u201354.7%), PA\/I38M in 3 specimens (variant call frequencies: 6.8\u201335.4%), and co-detection of both PA\/I38T and PA\/I38M in 1 specimen (PA\/I38T: 10.4%; PA\/I38M: 14.4%). Among the 11 A(H3N2) cases with PA substitutions detected after baloxavir treatment, the median fever duration after baloxavir administration was 33 hours (range: 0\u2013128 hours).<\/p>\n As NGS analysis is currently being prioritized for specimens in which the PA\/I38T substitution was identified by cycling-probe RT-PCR, the actual frequency of PA substitution detection may be overestimated.<\/p>\n NGS analysis was performed for 8 A(H3N2) specimens collected at follow-up after oseltamivir treatment. In the 1 specimen for which analysis was successful, no NA gene substitutions associated with reduced susceptibility to oseltamivir were detected by NGS (Table 4).<\/p>\n","protected":false},"excerpt":{"rendered":"
\nBetween October 2025 and February 17, 2026, we conducted a study in Japan to monitor circulating influenza strains and the emergence of antiviral resistance. The study focused on detecting substitutions in the PA gene associated with reduced susceptibility to baloxavir and substitutions in the NA gene associated with reduced susceptibility to neuraminidase inhibitors, including oseltamivir.<\/p>\n
\nOutpatient medical institutions in eight prefectures participated in this study. Upper respiratory tract samples were collected as initial samples from patients who visited these institutions with suspected influenza symptoms (influenza-like illness, ILI), such as fever, cough, sore throat, and fatigue. These patients were confirmed positive for influenza A or B using rapid diagnostic tests (RDTs). For patients treated with baloxavir, upper respiratory tract samples were also collected during follow-up visits 3\u20137 days after treatment. All clinical specimens were sent to Niigata University, where real-time RT-PCR was performed for influenza virus detection, type\/subtype determination, detection of the PA\/I38T substitution (for A(H1N1)pdm09 and A(H3N2)), and detection of the NA\/H275Y substitution (for A(H1N1)pdm09). Next-generation sequencing (NGS) and Sanger sequencing were used to confirm PA and NA substitutions.<\/p>\n
\nAmong 173 baseline specimens, influenza A(H1N1)pdm09 was detected in 0 specimens (0%), influenza A(H3N2) in 158 specimens (91.3%), unsubtyped influenza A in 2 specimens (1.2%), influenza B\/Victoria in 7 specimens (4.0%), and influenza B with undetermined lineage in 0 specimens (0%). No mixed A and B infections were identified (0.0%), and 6 specimens (3.5%) were RT-PCR negative (Table 1).<\/p>\n
\nFollow-up (post-treatment) specimens were collected from 87 patients. Among them, 49 patients (48 A(H3N2) and 1 B\/Victoria) provided upper respiratory tract specimens 3\u20137 days after the start of baloxavir treatment (Table 1).<\/p>\n