A new COVID-19 subvariant called NB.1.8.1 — nicknamed "Nimbus" — has become the dominant strain circulating in the United States, accounting for an estimated 43% of COVID cases as of late June 2026 according to CDC genomic surveillance data. Multiple states are reporting "high" or "very high" COVID viral levels in wastewater, concentrated in the South and West.
Wastewater surveillance detects COVID viral particles in sewage before those infections are reflected in clinical test counts or hospital data — typically providing a one- to two-week early warning of rising community transmission. For high-risk individuals in the states listed above, the wastewater signal represents an early warning, not yet a clinical surge. Acting on it now offers the best window to prepare.
Overall national wastewater activity remains low. But the geographic pattern — concentrated in the South and West — is consistent with the CDC's summer outlook, which identified these regions as most likely to see early transmission increases.
Why This Matters
For most healthy adults, Nimbus is expected to cause symptoms similar to other recent Omicron strains: a few days of sore throat, fatigue, fever, and upper respiratory congestion — with some reports describing an unusually severe "razor blade throat" sensation. The CDC and WHO have classified NB.1.8.1 as a Variant Under Monitoring — not a Variant of Concern — and available data from Asia, where the variant drove earlier waves, suggest it does not cause a higher rate of severe illness than prior Omicron variants.
But for specific populations — adults 65 and older, immunocompromised individuals, and people with multiple chronic conditions — the consequences of COVID infection remain meaningfully elevated, regardless of the variant's general severity profile. For these groups, the wastewater signal is a reason to verify their vaccine status and confirm they have a clear path to treatment if they become infected.
What We Know So Far
NB.1.8.1 surpassed all other circulating variants in late June 2026 to become the dominant U.S. strain, accounting for approximately 43% of COVID cases according to CDC genomic surveillance data. 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.
The variant is a descendant of the JN.1 Omicron lineage. Available data from Asia suggest NB.1.8.1 is highly transmissible 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 CDC states that its precision in variant proportion reporting is currently "low" due to limited sequencing data, so the exact 43% figure may shift as more sequences are submitted. Nationally, emergency department visits and clinical case indicators have not yet shown a major spike.
Where the Risk Is Highest
States currently reporting high or very high COVID wastewater viral activity include Alabama, Alaska, California, Delaware, Florida, Hawaii, Kentucky, Louisiana, and Texas. Residents of major metropolitan areas within these states — including Los Angeles, Houston, Miami, New Orleans, and Honolulu — should be particularly aware of local transmission trends.
Wastewater monitoring data is available by state through the CDC's National Wastewater Surveillance System at cdc.gov/wastewater. Data is updated weekly and provides a useful early indicator of local trends.
What Doctors and Experts Say
The most important piece of clinical guidance for the current early summer period is consistent across infectious disease experts: the window before a wave peaks is the right time for high-risk individuals to act, not the time to wait and see.
Clinicians are advising high-risk patients — particularly adults 65 and older — to verify whether their last COVID vaccine was more than 12 months ago. The 2025–2026 updated COVID vaccines targeting the LP.8.1 variant are available for adults 65 and older and for people ages 12–64 with at least one underlying condition.
For patients who develop COVID infection, Paxlovid (nirmatrelvir-ritonavir) remains the primary antiviral treatment recommended for high-risk individuals — but it must be started within five days of symptom onset to be most effective. Having a plan ahead of time makes faster treatment initiation possible.
What the Evidence Shows — and What It Does Not
MedicalDaily Evidence Check
- Variant: NB.1.8.1 ( "Nimbus" ), a JN.1 Omicron lineage descendant
- 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; multiple states reporting high or very high levels concentrated in the South and West
- Severity profile: Available data suggests no increased severity compared to other recent Omicron variants; U.S. clinical severity data still accumulating
- Note: CDC precision is "low" due to limited sequencing data; figures may shift
Who Faces the Greatest Risk?
- Adults 65 and older — regardless of vaccination status
- Immunocompromised individuals, including transplant recipients, people on cancer treatment, and those on biologics
- Adults with multiple chronic conditions (cardiovascular disease, chronic kidney disease, diabetes, COPD)
- People who have not received a COVID vaccine in more than 12 months
- Residents of high-activity states who fall into any of the above categories
Symptoms and Warning Signs to Watch For
Nimbus symptoms appear to be broadly similar to other recent Omicron variants and may include sore throat (some reports describe an unusually severe "razor blade" sensation), fever or chills, fatigue and body aches, congestion or runny nose, headache, and cough.
Seek urgent medical care for: difficulty breathing, chest pain, persistent confusion, inability to stay awake, or any rapidly worsening condition. High-risk individuals who test positive should contact a clinician immediately to discuss antiviral treatment — do not wait for symptoms to worsen.
What You Can Do Now
- Check COVID vaccine status. If your last COVID vaccine was more than 12 months ago and you are in a high-risk category, discuss timing of an updated vaccine with your care provider.
- Stock rapid tests. Having COVID rapid tests at home allows you to test at symptom onset — critical for accessing Paxlovid within the five-day treatment window.
- Confirm access to Paxlovid. If you are high-risk, ask your primary care clinician to discuss a Paxlovid plan in advance.
- Monitor local wastewater data. Track your state's COVID wastewater levels weekly at the CDC NWSS to understand local trends before clinical data catches up.
- Wear masks in crowded indoor settings if you are high-risk and live in or are traveling to a high-activity state.
Cost and Access: What Patients Should Know
COVID vaccines are available at pharmacies, community health centers, and physician offices. For high-risk individuals 65 and older, vaccines are covered by Medicare. For adults 12–64 with underlying conditions, most private insurance plans cover updated COVID vaccines.
Paxlovid is covered by most insurance plans for eligible patients. For uninsured patients, the Pfizer patient assistance program and community health centers can help access antiviral treatment.
The Bottom Line
Nimbus (NB.1.8.1) is now the dominant COVID strain in the United States, and multiple states are already showing elevated wastewater viral activity consistent with early summer transmission increases. For most healthy adults, the risk profile is similar to other recent Omicron variants. But for adults 65 and older, immunocompromised individuals, and people with multiple chronic conditions, now is the right time to verify vaccine status, stock rapid tests, and have a treatment plan ready — before wastewater signals translate into clinical cases.
