COVID-19 Cases up for 8th Consecutive Week in Japan; Omicron Subvariant Nimbus Accounts for About 40% of Infections
COVID-19 Cases up for 8th Consecutive Week in Japan
Japan is grappling with a persistent rise in COVID-19 cases, marking the eighth consecutive week of increasing infections, according to the latest data from the Ministry of Health, Labor and Welfare. The surge, primarily fueled by the highly transmissible Omicron subvariant Nimbus, which accounts for approximately 40% of infections, has raised concerns among health officials and the public. As the nation navigates this latest wave, experts are urging heightened vigilance, particularly as the summer season and increased mobility during the Bon holidays may exacerbate the spread. This article delves into the key facts surrounding this ongoing public health challenge, exploring the characteristics of the Nimbus subvariant, regional trends, public health responses, and the broader implications for Japan’s pandemic management strategy.
The Surge in Cases: A National Overview
The Ministry of Health, Labor and Welfare reported on August 16, 2025, that the average number of COVID-19 cases per medical institution rose to 6.13 for the week of August 4–10, a 1.11-fold increase from the previous week’s 5.53 cases per institution. This data, derived from approximately 3,000 designated medical institutions nationwide, underscores the steady climb in infections since mid-June. The consistent week-on-week increase signals a robust wave, with the Omicron subvariant Nimbus identified as a primary driver. According to the Japan Institute for Health Security, Nimbus accounted for about 40% of infections in July, highlighting its significant role in the current surge.
The regional distribution of cases reveals notable disparities. Miyazaki Prefecture reported the highest infection rate, with 14.71 cases per institution, followed closely by Kagoshima Prefecture at 13.46 and Saga Prefecture at 11.83. The Kyushu region, encompassing these prefectures, has emerged as a hotspot, with experts attributing the elevated numbers to a combination of the subvariant’s transmissibility and increased social interactions during the summer. Other regions, including Tokyo and Osaka, have also seen upticks, with Tokyo reporting a 1.25-fold increase and Osaka a 1.40-fold increase in cases compared to the previous week.
This sustained rise follows a period of relative stability earlier in 2025, when Japan had downgraded COVID-19 to a Class V infectious disease, aligning its management with that of seasonal influenza. However, the emergence of Nimbus and its rapid spread have prompted renewed discussions about the need for targeted interventions to curb the wave without resorting to the stringent measures seen earlier in the pandemic.
Understanding the Nimbus Subvariant
The Omicron subvariant Nimbus, scientifically designated as NB.1.8.1, has garnered attention for its high transmissibility and distinct symptom profile. Unlike earlier Omicron subvariants such as BA.5 or XBB.1.5, Nimbus is noted for causing a severe sore throat, often described as a “razor blade throat” by those infected. This symptom, alongside more common signs such as fever, cough, headache, and fatigue, has made Nimbus particularly noticeable among patients. Health experts, including Nobuhiko Okabe, an advisor at the Kawasaki City Institute for Public Health, have emphasized the importance of recognizing these symptoms to facilitate early testing and isolation.
Nimbus’s dominance, accounting for approximately 40% of infections, is attributed to its ability to partially evade immunity from prior infections and vaccinations. The Japan Institute for Health Security notes that the subvariant’s spike protein mutations enhance its capacity to bypass neutralizing antibodies, a characteristic shared with other Omicron offshoots like XBB.1.5 and BA.2.86. However, there is no evidence to suggest that Nimbus causes more severe illness than previous variants. Hospitalization rates remain relatively low, with severe cases primarily affecting the elderly and those with underlying health conditions.
The subvariant’s rise coincides with a period of increased mobility, particularly during the Bon holidays in mid-August, a time when millions of Japanese travel to visit family or participate in cultural festivities. This heightened movement, coupled with relaxed public health measures, has likely facilitated Nimbus’s spread. Prof. Hiroyuki Kunishima of St. Marianna University School of Medicine highlights that waning immunity from vaccinations, especially among those who have not received boosters, increases susceptibility to infection. As more time passes since the last vaccination, individuals, particularly the elderly, are at greater risk of developing severe symptoms.
Regional Hotspots and Public Health Challenges
The Kyushu region’s prominence in the current wave underscores the uneven distribution of infections across Japan. Miyazaki, Kagoshima, and Saga prefectures have reported the highest per-institution case counts, prompting local authorities to enhance surveillance and testing efforts. In Miyazaki, for instance, health officials have expanded testing capacity at local clinics and urged residents to adopt preventive measures such as mask-wearing and handwashing. The region’s warm climate and outdoor activities during the summer may have contributed to gatherings that facilitated transmission, despite the government’s emphasis on ventilation.
Tokyo and Osaka, Japan’s largest urban centers, are also experiencing significant increases. Tokyo’s case count rose by 25% from the previous week, while Osaka saw a 40% jump, reflecting the challenges of managing outbreaks in densely populated areas. The capital’s Shinagawa station, a major transportation hub, has been described as “packed as usual” with commuters, highlighting the difficulty of enforcing social distancing in high-traffic areas. Similarly, Osaka’s bustling commercial districts have seen a return to pre-pandemic activity levels, complicating efforts to curb transmission.
Nationwide, the healthcare system remains under pressure but has not yet reached the critical levels seen during earlier waves, such as the BA.5-driven surge in July 2022, when daily cases exceeded 200,000. Hospital bed occupancy rates are currently manageable, with Tokyo and Osaka reporting rates of 25% and 26%, respectively, as of early August 2025. However, health experts warn that a prolonged wave could strain resources, particularly in regions with limited medical infrastructure.
Public Health Response and Recommendations
In response to the rising cases, the Ministry of Health, Labor and Welfare has reiterated calls for voluntary preventive measures. Nobuhiko Okabe emphasized the importance of mask-wearing in high-risk settings, such as crowded indoor spaces, and maintaining good hand hygiene. Ventilation remains a key recommendation, particularly as cooler temperatures in the coming months may reduce natural airflow in homes and public spaces. The ministry has also urged eligible individuals to receive booster shots, noting that updated vaccines targeting Omicron subvariants, including XBB.1.5, are believed to offer some protection against Nimbus.
Japan’s vaccination strategy has evolved significantly since the pandemic began. Free vaccinations, initiated in February 2021, have been expanded to include children as young as six months, with a focus on high-risk groups such as the elderly. The introduction of bivalent vaccines targeting BA.1 and BA.5 subvariants in 2022 and XBB.1.5 in 2023 has bolstered immunity, but uptake of boosters remains lower than desired. As of August 2025, only a fraction of the population has received the latest boosters, contributing to the current susceptibility to Nimbus.
The government has resisted reimposing strict measures such as lockdowns or restaurant closures, which were employed during earlier waves driven by subvariants like BA.5 and BA.2. Instead, the focus is on balancing public health with economic recovery. Japan’s low per capita COVID-19 death rate among OECD countries—246 per million, according to Our World in Data—has been attributed to widespread mask-wearing and a cautious public. However, compliance with voluntary measures has waned, with many Japanese resuming normal activities, including commuting on crowded trains and dining out.
The Role of Nimbus in the Global Context
The emergence of Nimbus aligns with global trends in COVID-19 variant dynamics. In the United States, the Omicron subvariant XFG, dubbed “Stratus,” has become predominant, driving a late-summer surge in cases. According to the CDC, Stratus shares similarities with Nimbus in its ability to evade immunity, though Nimbus remains the dominant strain in Japan. Both subvariants highlight the ongoing evolution of SARS-CoV-2, with mutations enabling greater transmissibility and immune escape.
Globally, the World Health Organization (WHO) has classified several Omicron subvariants, including XBB.1.5 and BA.2.75, as variants of interest or under monitoring. While Nimbus has not been designated as a variant of concern, its rapid spread in Japan underscores the need for continued genomic surveillance. The National Institute of Infectious Diseases (NIID) has prioritized sequencing to track Nimbus and other emerging subvariants, ensuring that public health responses remain informed by the latest data.
The global experience with Omicron subvariants provides valuable lessons for Japan. For instance, the BA.5 wave in 2022 demonstrated the importance of timely booster campaigns and targeted restrictions in high-risk areas. Japan’s decision to maintain a flexible approach, avoiding blanket lockdowns, reflects a shift toward treating COVID-19 as an endemic disease. However, the persistence of new subvariants like Nimbus challenges this strategy, requiring a delicate balance between public health and societal normalcy.
Societal and Economic Impacts
The ongoing wave has reignited concerns about Japan’s economic recovery, particularly in the tourism and hospitality sectors. In 2022, stringent border controls and restrictions on restaurants deterred foreign tourists, with only 10% of pre-pandemic visitor numbers recorded in July of that year. While borders have since reopened, the current surge has led some establishments to reinstate anti-virus measures, such as limiting group sizes or refusing foreign guests. These precautions, while aimed at curbing transmission, risk undermining Japan’s efforts to revitalize tourism ahead of events like the Osaka Kansai Expo 2025, which expects 28.2 million visitors.
Public sentiment, as reflected in social media posts, reveals a mix of fatigue and skepticism. Some X users have dismissed Nimbus’s symptoms as akin to a common cold, questioning media portrayals of its severity. Others have expressed frustration with ongoing restrictions, with one user sarcastically asking, “Who has ever swallowed a razor blade?” in reference to the reported sore throat symptom. These sentiments highlight the challenge of maintaining public cooperation after years of pandemic-related disruptions.
Lessons from Past Waves
Japan’s experience with previous Omicron subvariants offers insights into managing the current wave. The BA.5 surge in July 2022, which saw a record 200,975 daily cases, was driven by a combination of high transmissibility and relaxed restrictions. The government’s decision to avoid lockdowns during that wave, focusing instead on voluntary measures and vaccinations, helped mitigate economic damage but led to significant strain on the healthcare system. Similarly, the BA.2 wave in early 2022 highlighted the importance of genomic sequencing to track subvariant spread, a practice that remains critical with Nimbus.
The downgrading of COVID-19 to a Class V infectious disease in May 2023 marked a turning point, shifting the focus to individual responsibility and targeted medical interventions. However, the resurgence of cases underscores the need for ongoing vigilance, particularly as new subvariants emerge. The NIID’s analysis of past waves, such as the BA.5-dominant period, revealed a shorter incubation period (2.6 days) compared to Delta (3.7 days), suggesting that Nimbus may follow a similar pattern, necessitating rapid response measures.
Conclusion: Navigating the Nimbus Wave and Beyond
The eighth consecutive week of rising COVID-19 cases in Japan, driven by the Omicron subvariant Nimbus, marks a critical juncture in the nation’s pandemic response. The key facts—6.13 cases per institution, a 1.11-fold increase from the previous week, and Nimbus’s 40% share of infections—paint a picture of a resilient virus that continues to challenge public health strategies. The Kyushu region’s prominence as a hotspot, alongside upticks in Tokyo and Osaka, underscores the need for localized interventions tailored to regional dynamics. While hospitalization rates remain low, the potential for strain on healthcare resources looms, particularly as the population’s immunity wanes.
Nimbus’s characteristics, including its severe sore throat and immune-evading mutations, highlight the ongoing evolution of SARS-CoV-2. The subvariant’s spread during the Bon holidays reflects the interplay between human behavior and viral dynamics, a recurring theme in Japan’s pandemic experience. Public health responses, centered on voluntary measures like mask-wearing and booster shots, aim to balance containment with economic recovery. However, declining compliance and vaccine fatigue pose significant hurdles, as evidenced by public sentiment on platforms like X.
Looking forward, Japan must draw on lessons from past waves to navigate this surge effectively. The success of previous vaccination campaigns, coupled with advancements in genomic surveillance, provides a foundation for targeted interventions. The government’s reluctance to reimpose strict measures reflects a broader shift toward endemicity, but this approach requires robust public engagement to succeed. Enhancing booster uptake, particularly among the elderly, and maintaining flexible testing and surveillance systems will be critical to mitigating Nimbus’s impact.
Globally, Japan’s experience with Nimbus offers insights into managing emerging subvariants in a post-pandemic world. The WHO’s monitoring of variants like XBB.1.5 and Stratus underscores the need for international cooperation in tracking and responding to SARS-CoV-2’s evolution. As Japan prepares for major events like the Osaka Kansai Expo, ensuring a resilient healthcare system and informed public will be paramount.
In conclusion, the Nimbus-driven wave is a reminder that COVID-19 remains a dynamic challenge. By leveraging data-driven strategies, fostering public cooperation, and maintaining vigilance, Japan can navigate this surge and prepare for future variants. The path forward lies in balancing caution with normalcy, ensuring that the lessons of the past inform a sustainable approach to an ever-evolving virus.
