As the peak summer travel season approaches in the United States, a new COVID-19 variant, NB.1.8.1, colloquially referred to as “Nimbus,” is rapidly spreading worldwide. This highly contagious omicron subvariant, which triggered surges across Asia this spring, is now responsible for over one-third of COVID cases in the U.S., according to recent data.
Nimbus was first identified in the U.S. in late March via airport screening programs for international travelers. Since its detection, this fast-moving variant has been reported across more than a dozen states, raising concerns among public health officials about the potential for rising cases.
On May 23, the World Health Organization (WHO) categorized NB.1.8.1 as a “variant under monitoring” due to its swift global spread and notable mutations in its spike protein, which may enhance its transmissibility.
The moniker “Nimbus” was coined in late May by T. Ryan Gregory, a Canadian evolutionary biologist known for coining other popular variant names. His insights into viral evolution have been valuable in understanding how these mutations could impact public health.
While Nimbus is contributing to an increasing proportion of COVID cases in the U.S., current trends show relative stability overall. As of June 7, test positivity stood at 3%, a slight increase of 0.2% from the previous week. Hospitalizations due to COVID are also decreasing, based on the most recent data from the U.S. Centers for Disease Control and Prevention (CDC).
Further, the CDC reported that the level of COVID viral activity in wastewater is currently “low” nationwide. However, experts warn that this situation may evolve in the upcoming months. Historically, the U.S. has experienced spikes in COVID-19 cases each summer since 2020, and the emergence of a new variant like Nimbus has fueled concerns of another surge.
Nimbus: The Second-Most Common Variant in the U.S.
In the two-week period ending June 7, NB.1.8.1 comprised an estimated 37% of COVID cases in the U.S., a significant jump from 15% in the preceding two-week period, according to CDC data. The leading variant at this time, LP.8.1, accounted for 38% of cases.
It’s worth noting that the precision of this reporting is “low” due to insufficient sequencing data, which is crucial for tracking the emergence of new variants. Although the CDC has not yet released detailed regional data on NB.1.8.1, sequences shared with the Global Initiative on Sharing All Influenza Data (GISAID) indicate that Nimbus has been detected in at least 14 states, including:
Arizona, California, Colorado, New Jersey, New York, Hawaii, Illinois, Maryland, Massachusetts, Ohio, Rhode Island, Vermont, Virginia, and Washington.
Globally, as of May 18, Nimbus has been reported in at least 22 countries and is responsible for 10.7% of COVID cases worldwide, according to the WHO. With its rapid spread, public health experts anticipate that cases driven by NB.1.8.1 in the U.S. will likely continue to rise.
Is the New COVID Variant More Transmissible?
SARS-CoV-2, the virus responsible for COVID-19, constantly mutates, leading to new variants that may spread more easily or evade existing immunity. Historically, all descendants of the original Omicron variant have shown high levels of contagion.
NB.1.8.1 stems from the recombinant omicron variant XDV.1.5.1 and possesses several mutations in its spike protein. These mutations could potentially enhance its ability to transmit and evade COVID antibodies from prior infections or vaccinations.
A preliminary study published in May, which has yet to undergo peer review, suggested that the mutations in NB.1.8.1 might enable the virus to bind more effectively to human cells, thus increasing its transmissibility and raising the “potential for future dominance” over other variants.
While evidence suggests that Nimbus likely has enhanced transmissibility, experts caution against jumping to conclusions about how it compares to existing strains.
Importantly, there is currently no indication that Nimbus results in more severe illness compared to other variants. The WHO has classified the global risk posed by NB.1.8.1 as “low.”
Will Nimbus COVID Cases Continue to Rise?
Whether NB.1.8.1 will overtake dominant variants and trigger a summer surge in the U.S. remains uncertain. “It’s hard to predict whether this variant or another will take off and cause a wave… but it’s likely that it won’t be as severe as what we experienced before 2023,” Dr. Albert Ko, a professor of public health, epidemiology, and medicine at Yale School of Public Health, shared with TODAY.com.
Immunity levels in the U.S. are relatively high, with an estimated 90% of Americans having been exposed to COVID-19, either via infection or vaccination. However, immunity does wane over time.
As Dr. Ko noted, “The longer we get from the last surge or vaccination campaign, the more susceptible the population becomes.” Alongside the emergence of variants such as NB.1.8.1 that may evade immunity, the rise in summer travel and social gatherings could facilitate rapid viral spread.
The CDC has observed that COVID-19 can surge at various times throughout the year. In the U.S., peaks generally occur twice: once in winter and again during the summer months from June to August. However, the timing and scale of these summer surges can differ significantly.
Based on historical trends, experts anticipate an increase in COVID-19 cases in the coming months, though it remains uncertain how this summer will unfold. “We may see an upswing this summer, as in previous years, but it’s expected to be smaller compared to winter surges,” Dr. Ko added.
New COVID Variant NB.1.8.1 Nimbus Symptoms
Preliminary observations indicate that the symptoms associated with Nimbus are similar to those caused by recent omicron strains, which may include:
- Sore throat
- Cough
- Runny nose or congestion
- Shortness of breath
- Fever or chills
- Headache
- Body aches
- Fatigue
- New loss of sense of smell or taste
While anyone can contract COVID-19, certain groups are at greater risk for severe illness, including individuals over age 65, those who are immunocompromised, or those with underlying health conditions, as noted by the CDC.
Should You Get the COVID Vaccine?
The COVID vaccine remains a vital tool in protecting against severe illness, hospitalization, and death. With recent modifications to vaccination guidelines, you may be wondering about the appropriateness of receiving another dose.
The CDC continues to recommend the 2024-2025 COVID-19 vaccines for most adults aged 18 and older.
Experts advise that those in high-risk groups consider vaccination, especially if it’s been six months since their last shot, and to remain vigilant about receiving updated vaccines in preparation for the upcoming winter season.
According to the WHO, existing COVID vaccines are expected to maintain efficacy against the NB.1.8.1 variant.
The 2025-2026 COVID vaccines, aiming at the LP.8.1 variant, are slated for release this fall. However, new guidelines from Health and Human Services Secretary Robert F. Kennedy Jr. will limit access to these vaccines for adults 65 and older, and for those aged 12-64 with underlying conditions. Routine vaccinations for healthy children and pregnant women are no longer recommended by the CDC, though many practitioners remain skeptical of this decision.
If you have questions or concerns regarding the COVID-19 vaccine, it is always best to consult with your physician.
Protecting oneself and others from COVID-19 involves testing when exhibiting symptoms, isolating during sickness, wearing masks indoors, and practicing social distancing.
While the government no longer distributes complimentary at-home COVID-19 testing kits, there are still avenues for obtaining free or low-cost testing options.
