September 15, 2025, reports indicate a resurgence of COVID-19 infections across Japan this summer, driven by the Nimbus variant (NB.1.8.1), an Omicron sublineage. The variant, first detected globally in January 2025, has been designated a “variant under monitoring” by the World Health Organization (WHO) due to its increased transmissibility, though it does not appear to cause more severe illness than previous strains. According to the National Institute of Infectious Diseases (NIID), formerly known as the National Institute for Health Crisis Management Research, approximately 80% of sequenced cases in July 2025 were Nimbus or its related lineages.
A 27-year-old woman visiting a clinic in Tokyo reported severe symptoms, including a sharp headache and intense throat pain described as “like a pointed object stabbing.” The physician confirmed a positive antigen test and attributed the symptoms to Nimbus, noting its characteristic throat irritation, often likened to a “razor blade” sensation. Other common symptoms include fever, cough, fatigue, muscle pain, nasal congestion, diarrhea, and nausea, with an incubation period similar to prior variants.
Infection numbers have risen steadily. Per the Ministry of Health, Labour and Welfare (MHLW), the average weekly cases per medical institution increased from 0.9 in the week ending June 15 to 8.73 by August 24, marking 10 consecutive weeks of growth. The week ending September 7 saw 8.12 cases, a slight decline for two weeks but still elevated. This uptick coincides with relaxed hygiene practices and record-breaking heat, which has discouraged medical visits.
A survey by Shionogi & Co., a major pharmaceutical firm, revealed that about one in five respondents avoided seeking care due to the heat, with an average delay of 3.2 days from symptom onset to consultation. For those in their 70s and 80s, this delay extended to 4.3 days, the longest among age groups. Dr. Koichi Hirabata, an expert in COVID-19 sequelae, highlighted that over 35,000 deaths occurred in 2024 from the virus, emphasizing that delayed visits in hot weather can lead to severe progression in this highly transmissible disease.
The Nimbus variant emerged from recombinations of Omicron sublineages like XDE and JN.1, featuring mutations in the spike protein that enhance binding to human ACE2 receptors, boosting infectivity by up to 2.5 times compared to earlier strains like LP.8.1. It has spread to over 20 countries, accounting for 40-43% of U.S. cases by mid-2025 and driving summer waves in Asia, including Japan. Current vaccines, including the 2024-2025 formulations targeting JN.1 and updated LP.8.1 versions approved in August 2025, remain effective against severe outcomes, with no reported resistance to antivirals like Paxlovid. Japan’s surveillance system, reorganized in April 2025, continues to track variants, while experts note that summer surges, like this one, are exacerbated by indoor crowding during heatwaves and reduced ventilation. On X, recent discussions highlight public awareness of Nimbus’s symptoms and calls for renewed precautions, with users sharing tips on early testing and hydration to mitigate risks.
source : Yahoonews
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