A COVID-19 subvariant that drove surges in China and Southeast Asia earlier this year has now become the dominant strain in the United States and is fueling rising emergency department visits across 27 states, according to the latest federal surveillance data. The variant, known as NB.1.8.1 and informally called "Nimbus," accounts for an estimated 43% of all sequenced U.S. COVID cases as of late June 2026, according to CDC genomic surveillance data. Wastewater levels classified as "high" or "very high" have been confirmed in Alabama, Alaska, California, Florida, Hawaii, Kentucky, Louisiana, and Texas, and public health officials are urging high-risk individuals to verify their vaccination status before summer travel reaches its peak.
Why This Matters
Summer COVID waves have occurred every year since 2020. The pattern of a new dominant variant emerging alongside rising wastewater signals in multiple states is consistent with the early stages of that annual summer cycle. For most vaccinated, healthy adults, Nimbus appears to cause a manageable illness: a sore throat, fatigue, and several days of upper respiratory symptoms.
The picture is different for older adults, immunocompromised individuals, and people who have not received an updated COVID vaccine in the past six months. Rising ER visits across 27 states signal that some portion of the population is being hospitalized at rates that warrant attention, even if overall national activity remains moderate. Summer travel, with crowded airports, hotels, and indoor venues, creates accelerated conditions for transmission in both high-risk and lower-risk individuals.
What We Know So Far
NB.1.8.1 is a descendant of the JN.1 Omicron lineage. It was first detected globally in early 2025 and identified in the United States through airport screening programs in March 2026. By late June, CDC nowcast data showed it had surpassed all other circulating variants to account for an estimated 43% of U.S. COVID cases.
The World Health Organization has classified NB.1.8.1 as a "Variant Under Monitoring," one classification level below a Variant of Interest and two levels below a Variant of Concern. This designation reflects elevated surveillance attention but does not indicate evidence of increased severity or vaccine escape sufficient to require an emergency response.
Emergency department visit data from the CDC's mid-July 2026 respiratory illness summary confirms that visits are rising in 27 states, driven primarily by the South and West. The CDC's National Wastewater Surveillance System currently shows "high" or "very high" COVID viral concentrations in Alabama, Alaska, California, Florida, Hawaii, Kentucky, Louisiana, and Texas.
Where the Risk Is Highest
Wastewater surveillance provides an early warning that typically precedes clinical test counts and hospital data by one to two weeks. The states currently showing elevated wastewater signals are where community transmission is building most actively, concentrated in the South and West.
Within those states, the highest-density urban areas carry the greatest transmission risk: Los Angeles and San Francisco in California; Houston, Dallas, and San Antonio in Texas; Miami and Jacksonville in Florida; and New Orleans and Baton Rouge in Louisiana. These metro areas combine high population density, active tourism, and major transportation hubs.
States not yet showing elevated wastewater signals may still see transmission increases over the coming weeks as the wave moves through the country. The CDC's 2026 Summer Outlook had identified the South and West as the most likely regions for early summer COVID activity, and current data are tracking with that forecast.
What Doctors and Experts Say
Data from Asia, where NB.1.8.1 drove earlier surges, indicate that the variant does not appear to cause a higher rate of severe illness than prior Omicron variants. The WHO has noted that currently approved COVID vaccines are expected to remain protective against NB.1.8.1 for the outcomes that matter most: hospitalization and severe disease.
The current 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 2026 vaccine cycle, but no announcement has been made as of mid-July. Getting the most recently available updated vaccine, if you have not had one in the past six months, remains the most evidence-supported step for reducing serious illness risk.
One notable feature of NB.1.8.1 reported by clinicians in countries where it circulated earlier is a distinctive, severe sore throat sometimes described as feeling like razor blades. Readers experiencing that symptom should consider COVID testing before assuming strep or another cause.
What the Evidence Shows and What It Does Not
MedicalDaily Evidence Check
- Surveillance type: CDC genomic sequencing (variant proportion) and National Wastewater Surveillance System
- Variant proportion: 43% of sequenced U.S. cases attributed to NB.1.8.1 as of late June 2026. The CDC notes precision is currently "low" due to limited sequencing data; the proportion may shift
- WHO classification: Variant Under Monitoring (not a Variant of Concern)
- Severity data: Available international data do not show higher severe illness rates than prior Omicron variants
- What it does not prove: That this wave will be as large as prior summer surges; wastewater is a leading indicator, not a confirmed case count
- What readers should know: High-risk individuals should verify vaccine status and ensure access to antiviral treatment (Paxlovid) before becoming ill
Who Faces the Greatest Risk?
Public health officials consistently identify the same high-risk populations for severe COVID outcomes regardless of the variant:
- Adults 65 and older
- Immunocompromised individuals, including people on chemotherapy, with HIV, or taking immunosuppressant medications
- People with multiple chronic conditions, including heart disease, diabetes, chronic lung disease, and kidney disease
- People who have not received an updated COVID vaccine in the past six months
- Residents of, or travelers to, states with "high" or "very high" wastewater signals who have elevated personal risk factors
- Pregnant people, for whom COVID infection carries a heightened complication risk
For healthy, vaccinated adults without underlying conditions, Nimbus is expected to produce symptoms comparable to prior Omicron variants.
Symptoms and Warning Signs to Watch For
NB.1.8.1 symptoms reported in affected countries include:
- Severe sore throat (described as unusually intense, a distinguishing feature)
- Fever and chills
- Fatigue
- Headache
- Cough
- Congestion or runny nose
- Muscle aches
- Loss of taste or smell (less common with Omicron descendants, but reported in some cases)
High-risk individuals who develop these symptoms should test for COVID promptly, because antiviral treatment with Paxlovid is most effective when started within five days of symptom onset. Anyone experiencing trouble breathing, persistent chest pressure, new confusion, inability to stay awake, or bluish lips or face should seek emergency care immediately.
What You Can Do Now
- Check your COVID vaccination status. If you have not received an updated COVID vaccine in the past six months, consult your health care provider about whether getting one now makes sense for your situation.
- Confirm access to COVID testing before you need it: at-home tests, pharmacies, and local health department sites.
- If you are high-risk, speak with your doctor in advance about eligibility for Paxlovid, so you can begin treatment quickly if you test positive.
- In crowded indoor settings during summer travel, consider masking if you are high-risk or traveling with someone who is.
- Track your state's COVID wastewater levels at the CDC NWSS dashboard , which updates weekly and provides an early warning before clinical data catches up.
- If you develop a severe sore throat with fever after travel, consider COVID testing before assuming it is strep.
Cost and Access: What Patients Should Know
Paxlovid, the primary antiviral used to reduce severe COVID illness risk, is available through most pharmacies with a prescription. As of 2026, it is covered by most insurance plans and Medicare Part D, though out-of-pocket costs for uninsured patients can be significant. Patients without insurance should ask their provider or pharmacist about any available patient assistance programs.
Updated COVID vaccines remain available at most pharmacies and through primary care providers, with coverage under most insurance plans and Medicare. Community health centers and local health departments often offer vaccines at low or no cost for uninsured individuals.
What Happens Next
The CDC updates its COVID wastewater and variant proportion data weekly. The Summer Outlook will be updated as surveillance data accumulates through July and August. Health authorities are monitoring whether the fall 2026 COVID vaccine formulation should target NB.1.8.1 or a newer variant; decisions on fall vaccine strain composition typically come in late summer. MedicalDaily will report on significant changes in wastewater trends, severity data, or vaccine guidance as they emerge.
The Bottom Line
The Nimbus variant is now the dominant COVID strain in the United States, and emergency department visits are rising in 27 states. For most healthy, vaccinated Americans, this wave is likely to produce an uncomfortable but manageable illness. For older adults, immunocompromised individuals, and anyone without an updated vaccine, this summer wave is a reason to act now: verify vaccine status, confirm access to testing and Paxlovid, and stay alert to symptoms that may warrant prompt evaluation.
