A COVID subvariant that drove surges in China, Singapore, and parts of Southeast Asia has now become the dominant strain circulating in the United States — and while overall national activity remains moderate, wastewater surveillance shows rising viral levels in 10 states, concentrated in the South and West.
The variant, known as NB.1.8.1 and informally called "Nimbus," is a descendant of the JN.1 Omicron lineage. By the two-week period ending late June 2026, it accounted for an estimated 43% of COVID cases in the United States, according to CDC genomic surveillance data. NB.1.8.1 surpassed all other circulating variants in that period to become the dominant U.S. strain.
The CDC's 2026 Summer Outlook identified the South and West as the most likely regions for a summer COVID surge, citing lower recent immunity in populations that had limited COVID exposure last winter.
Why This Matters
Three consecutive years of summer COVID surges since 2022 have established a recurring seasonal pattern in the United States. Each summer, waning immunity from winter infections — combined with increased indoor gatherings in air-conditioned spaces, summer travel, and a new circulating variant — has driven a wave of illness that peaked in late July or August.
NB.1.8.1 fits the conditions for another such wave. It spread rapidly in Asia, including approximately 60% weekly growth rates in Singapore, and has now established itself as the dominant U.S. strain. Available data from the World Health Organization, which classifies NB.1.8.1 as a "Variant Under Monitoring," and from the ECDC indicate the variant does not appear to cause more severe disease than other recent Omicron sublineages. But transmissibility — not severity — is what drives surge dynamics.
What We Know So Far
From CDC wastewater surveillance and genomic data:
- NB.1.8.1's U.S. proportion : Approximately 43% of all COVID cases as of late June 2026; now the dominant U.S. strain
- National wastewater level : "Low" nationally per the CDC's most recent update through the week ending approximately June 28, 2026
- States with "high" or "very high" wastewater COVID levels : Alabama, Alaska, California, Delaware, Florida, Hawaii, Kentucky, Louisiana, and Texas
- First U.S. detections : Through international airport screening in March 2026; originally identified in travelers arriving from China, France, Japan, the Netherlands, Spain, South Korea, Taiwan, and Thailand
- U.S. states with early case sequencing : California, Hawaii, New York, Ohio, Rhode Island, Virginia, and Washington
- WHO classification : Variant Under Monitoring (VUM) — not a Variant of Concern
The CDC no longer tracks total confirmed U.S. case counts, making wastewater surveillance the primary window into community-level COVID activity.
Where the Risk Is Highest
Based on CDC and WastewaterSCAN monitoring, states with the highest current COVID viral activity in wastewater include Alabama, Alaska, California, Delaware, Florida, Hawaii, Kentucky, Louisiana, and Texas. These states correspond to the CDC's Summer Outlook prediction that the South and West would be most susceptible to a summer surge.
The mechanism is straightforward: parts of the South and West experienced lower COVID transmission last winter, meaning that populations in those regions have had less recent immune priming from natural infection. As immunity wanes from older infections and vaccinations, susceptibility to NB.1.8.1 increases.
Major metro areas within high-risk states include Los Angeles, San Francisco, San Diego, Miami, Tampa, Jacksonville, Houston, San Antonio, Dallas, New Orleans, and Louisville.
Andy Pekosz, Ph.D., a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health, noted in media comments that SARS-CoV-2 appears to adapt better to hot, humid weather than other respiratory viruses, contributing to summer seasonality patterns.
What Doctors and Experts Say
"We don't see any significant upward trends happening yet, but we'll continue to watch this closely," said Marlene Wolfe, Ph.D., principal investigator at WastewaterSCAN, in an interview with TODAY earlier in June, referencing conditions at that time — before NB.1.8.1 became dominant.
Eduardo Colzani, M.D., M.P.H., Ph.D., ECDC's head of respiratory viruses, noted in a public statement that NB.1.8.1 "doesn't appear to pose a greater threat than other Omicron descendants" — a meaningful reassurance for clinical severity, though not for transmissibility.
The CDC has identified "razor blade throat" as a distinctive reported symptom of NB.1.8.1 in some patients — a description of severe sore throat that has been noted by clinicians treating early Nimbus cases.
What the Evidence Shows — and What It Does Not
NB.1.8.1 is categorized as a Variant Under Monitoring — a classification below Variant of Interest and well below Variant of Concern. This means the WHO and CDC are watching it closely but have not identified evidence of enhanced disease severity, vaccine escape sufficient to cause widespread serious illness in vaccinated populations, or other features that would require an emergency response.
The available data from Asia suggest NB.1.8.1 is highly transmissible — capable of driving large waves — but does not appear to cause a higher rate of severe illness, hospitalization, or death than LP.8.1 or other recently circulating Omicron strains.
The uncertainty is real: the variant is new enough that extensive clinical outcome data from a U.S. population context is not yet available. Surveillance will clarify the severity picture over the coming weeks.
MedicalDaily Evidence Check
- Variant : NB.1.8.1 (Nimbus), a descendant of JN.1/Omicron
- U.S. dominance : ~43% of sequenced cases as of late June 2026
- WHO classification : Variant Under Monitoring (not Variant of Concern)
- Wastewater activity : Low nationally; 10 states reporting high or very high levels
- Severity profile : Available data suggests no increased severity vs. other recent Omicron variants
- Key limitation : U.S. clinical severity data is still accumulating; severity picture may evolve
- What readers should know : Current COVID vaccines are expected to remain protective against serious illness from NB.1.8.1; people in high-risk groups should discuss their vaccine status with a provider
Who Faces the Greatest Risk?
COVID risk for severe outcomes continues to be concentrated in:
- Adults 65 and older
- People with weakened immune systems (cancer treatment, organ transplant, HIV, certain medications)
- Individuals with uncontrolled diabetes, heart disease, chronic kidney disease, or obesity
- Pregnant people
- Residents of nursing homes and long-term care facilities
- Unvaccinated adults who have not had a recent COVID infection
For healthy younger adults, NB.1.8.1 is expected to produce illness similar to other recent Omicron strains — typically a few days of upper respiratory symptoms, fatigue, and fever.
Symptoms and Warning Signs to Watch For
Symptoms reported in NB.1.8.1 cases include:
- Severe sore throat (described by some patients as "razor blade" sensation)
- Fever
- Fatigue and body aches
- Cough
- Nasal congestion and runny nose
- Headache
Loss of taste and smell has been less common with Omicron subvariants compared to earlier COVID strains, though some cases still occur.
Most healthy adults will recover at home with rest, hydration, and over-the-counter fever management. Seek medical care for: difficulty breathing, chest pain, confusion, inability to stay awake, persistent symptoms beyond 10 days, or any rapid worsening of condition.
What You Can Do Now
- Check your COVID vaccine status. The 2025–2026 updated vaccines are available for adults 65 and older and for people 12–64 with at least one underlying condition. If you are in a high-risk group and have not been vaccinated in the last 6 months, discuss timing with your provider.
- Monitor wastewater levels in your state at CDC's Wastewater Monitoring page . If your state shows high or rising viral activity, consider added precautions.
- Have COVID tests available at home. Rapid antigen tests can help you determine if symptoms are COVID and inform decisions about isolation and treatment.
- If you are in a high-risk group , ask your doctor about Paxlovid eligibility. Antiviral treatment is most effective when started within 5 days of symptom onset.
- If you develop a severe sore throat combined with fever and fatigue following summer travel or exposure, consider COVID testing before assuming it is strep or another cause.
Cost and Access: What Patients Should Know
Updated COVID vaccines for eligible adults are covered at no cost-sharing under most private insurance and Medicare. For uninsured individuals, vaccines are available at health departments and community health centers.
Paxlovid is covered by most insurance plans for eligible patients. For uninsured patients, Pfizer's patient assistance program may provide access at reduced or no cost — ask a pharmacist for current program availability.
At-home COVID tests are available at most pharmacies and retailers. Patients who need testing but cannot afford tests should contact their local health department, which may have free or low-cost testing resources.
What Happens Next
The CDC updates its COVID wastewater and variant proportion data weekly. The Summer Outlook will be updated as surveillance data accumulates over July and August. The 2025–2026 updated COVID vaccines target the LP.8.1 variant; health authorities are monitoring whether an NB.1.8.1-specific update will be needed for the fall vaccine cycle.
MedicalDaily will report on significant changes in wastewater trends, severity data, or vaccine guidance as they emerge.
The Bottom Line
NB.1.8.1 is now the dominant COVID strain in the United States, and 10 states are reporting elevated wastewater viral levels concentrated in the South and West. The available evidence suggests this variant does not cause more severe illness than other recent Omicron strains — but transmissibility is high, summer immunity waning is real, and the populations most likely to experience serious illness from COVID have not changed. People in high-risk groups should review their vaccine status now, before the summer wave fully develops.
