COVID-19 continues its unpredictable dance with humanity as two new subvariants, NB.1.8.1 and XFG, nicknamed “Nimbus” and “Stratus” respectively, are rapidly spreading across the globe. Though the pandemic may have slipped from the headlines, the virus itself has not stopped evolving, keeping scientists and health officials on their toes.
Identified in January 2025, Nimbus has swiftly gained ground, particularly across Asia and beyond. By late April 2025, it accounted for 11% of sequenced COVID-19 cases worldwide, a significant leap from just 2.5% a month earlier. Meanwhile, Stratus, first detected in Canada, has surged even faster, comprising 25% of cases in Europe by late May 2025 and showing a growing presence in India with over 200 cases reported by mid-June.
Despite their rapid spread, the World Health Organization (WHO) is cautiously optimistic. According to its Technical Advisory Group on Virus Evolution, as of May 23, 2025, available evidence does not suggest that Nimbus poses additional public health risks compared to other Omicron descendants. Similarly, Stratus, although spreading quickly, shows no signs of causing more severe illness.
Interestingly, these variants are products of viral recombination, a bit like Frankenstein’s monster stitched together from different parts. Nimbus descends from a complex lineage involving hybrids of earlier Omicron variants, carrying seven new mutations in its spike protein — the critical part that latches onto human cells — plus 20 other genetic changes. Stratus, a combination of two earlier variants, carries four key spike protein mutations.
Virological studies reveal that Nimbus binds strongly to the ACE2 receptor, the gateway the virus uses to invade human cells. This binding strength is even greater than that of Stratus and most other variants, potentially explaining why Nimbus infects cells so effectively. In fact, its infectivity may be up to 2.5 times higher than LP.8.1, the currently most prevalent COVID-19 variant worldwide. Stratus, on the other hand, excels in evading the immune system, with mutations that help it dodge antibodies, resulting in roughly a twofold reduction in antibody effectiveness compared to the dominant COVID variant. Nimbus shows a slightly lower, but still notable, 1.6-fold reduction.
Despite these immune evasions, vaccines remain a crucial line of defense. The WHO emphasizes that fully vaccinated individuals, particularly those with updated boosters, are still likely to be protected against severe illness, hospitalizations, and death caused by these variants.
On the clinical front, UK doctors have reported a peculiar symptom associated with Nimbus: a “razor blade” sensation in the throat. Dr. Naveed Asif, a general practitioner in London, described it as a sharp stabbing pain when swallowing, often at the back of the throat. Alongside this, patients have reported gastrointestinal symptoms such as nausea, vomiting, diarrhea, heartburn, bloating, constipation, and stomach pain, although these symptoms have yet to be confirmed by rigorous scientific studies. Indian doctors have also noted hoarseness as a common symptom among recent hospital patients.
The UK Health Security Agency (UKHSA) data reflects a concerning uptick in COVID cases and hospital admissions. In the week ending May 31, 2025, COVID-related hospitalizations rose by nearly 10% to 947 patients, while cases increased by 6.9% to 1,211 in the following week. Yet, Dr. Gayatri Amirthalingam of UKHSA reassures the public that there is no evidence Nimbus causes more severe disease or that vaccines are less effective against it.
Across the Atlantic, California is witnessing a similar rise in COVID activity, largely driven by the Nimbus subvariant. For the two-week period ending June 7, Nimbus accounted for an estimated 37% of coronavirus samples nationally, nearly matching LP.8.1's 38%. In California, Nimbus's presence surged from just 2% in April to a projected 55% of circulating virus by June 2025.
Wastewater surveillance paints a varied picture across Southern California, with moderate-to-high virus levels in Santa Barbara, low-to-moderate in Ventura and Los Angeles, low in Riverside, and high in San Bernardino. Los Angeles County reported a 13% increase in viral levels in wastewater for the 10 days ending May 30, compared to the previous period, with COVID surveillance tests positivity rising from 3.8% in early May to 5% recently. Northern California’s Santa Clara County also shows rising viral levels, with San José’s sewershed classified as “high.”
Healthcare experts like Dr. Elizabeth Hudson of Kaiser Permanente Southern California have noted these indicators as signs of increased COVID activity. Yet, emergency room visits related to COVID remain low, suggesting that while infections are rising, severe cases are not overwhelming hospitals.
However, the backdrop to this resurgence is complicated by recent shifts in vaccine policy under the Trump administration. Federal health officials have dropped longstanding recommendations for COVID vaccinations among healthy children and pregnant women. The Centers for Disease Control and Prevention (CDC) now offers “no guidance” for healthy pregnant women and advises parents of healthy children to consult healthcare providers before vaccination.
This policy shift has sparked criticism from medical organizations. The American College of Obstetricians and Gynecologists, the American Pharmacists Association, and a coalition of 30 health organizations, including the American Medical Association, have condemned these changes as lacking scientific basis and potentially harmful.
Moreover, Robert F. Kennedy Jr., nominated as Health Secretary, abruptly dismissed all members of the Advisory Committee on Immunization Practices (ACIP), a panel that has guided U.S. vaccine policy since 1964. Dr. Yvonne Maldonado, a Stanford infectious disease expert and one of the dismissed advisors, called this action unprecedented and expressed concern about the future clarity and integrity of vaccine recommendations.
The new immunization committee is scheduled to meet in late June 2025 to vote on recommendations for various vaccines, including COVID-19, HPV, flu, RSV, and meningococcal vaccines. Critics worry that the replacement members, some known for vaccine skepticism, could undermine vaccine confidence and access.
Governors of California, Oregon, and Washington have publicly condemned the dismissal of vaccine advisors, calling it “deeply troubling” and affirming their commitment to science-based vaccine policies. Meanwhile, the Los Angeles County Department of Public Health reassures residents that current vaccine recommendations remain in effect and insurance coverage for COVID-19 vaccines continues.
As the virus evolves and new variants like Nimbus and Stratus vie for dominance, the public faces a complex landscape of rising infections, evolving symptoms, and shifting vaccine policies. While the “razor blade” throat sensation may be a new and painful hallmark of Nimbus, the overarching message from health authorities remains consistent: vaccination, particularly with updated boosters, is vital to protect against severe outcomes.
In this ever-changing battle against COVID-19, vigilance, scientific rigor, and clear communication will be essential to navigate the challenges ahead.
