As summer approaches and travel activities peak, a new COVID-19 variant, NB.1.8.1—nicknamed ‘Nimbus’—is increasingly becoming a concern in the United States. This highly transmissible subvariant, a mutated form of omicron, has been linked to surges in various parts of Asia over the spring and is now accounting for over a third of new COVID cases in the U.S., according to the latest data.
Nimbus was first identified in the country in late March through airport screening programs for international travelers. Since its detection, it has spread to more than a dozen states, and health experts predict that cases will continue to rise in the coming months.
In May, the World Health Organization (WHO) designated NB.1.8.1 as a ‘variant under monitoring’ due to its rapid spread globally and its mutations that might enhance its transmissibility.
The term ‘Nimbus’ was coined in late May by T. Ryan Gregory, a Canadian evolutionary biologist known for creating catchy variant names, including the ‘FLiRT’ strains. Despite the growing proportion of cases attributed to NB.1.8.1, other COVID metrics such as test positivity and hospitalizations in the U.S. remain stable. As of June 6, the Centers for Disease Control and Prevention (CDC) reported low levels of COVID viral activity in wastewater across the nation.
However, experts warn that this trend may change as summer typically brings spikes in COVID-19 cases. Since 2020, there has been a consistent pattern of summer surges in COVID-19 cases, and with Nimbus circulating, concerns are mounting about a potential wave of new infections.
Recent data indicates that during the two-week period ending June 7, NB.1.8.1 was responsible for an estimated 37% of COVID-19 cases in the U.S., up from 15% in the preceding two weeks, according to the CDC. The variant currently follows LP.8.1, which is responsible for 38% of cases. The CDC acknowledges that the accuracy of its reporting is limited, primarily due to insufficient sequencing data that scientists rely on to trace emerging variants.
While the CDC refrained from detailing the regional spread of NB.1.8.1 in its latest update, sequences uploaded to the Global Initiative on Sharing All Influenza Data (GISAID) platform reveal that Nimbus has been identified in at least 14 states, including Arizona, California, Colorado, New Jersey, New York, Hawaii, Illinois, Maryland, Massachusetts, Ohio, Rhode Island, Vermont, Virginia, and Washington.
Moreover, the variant has been reported in at least 22 countries since May 18 and now constitutes 10.7% of global COVID-19 cases, per WHO data.
A notable question arises— is NB.1.8.1 more transmissible than previous variants? As the virus SARS-CoV-2 mutates constantly, new variants emerge with genetic modifications that can enable easier transmission or evade immunity from past infections or vaccinations. Historical evidence suggests that all descendants of the original Omicron variant are highly infectious.
NB.1.8.1, a descendant of the recombinant omicron variant XDV.1.5.1, possesses several mutations in its spike protein that may enhance its ability to transmit and evade antibodies formed through prior infections or vaccinations. A study published in May, which is currently awaiting peer review, indicates that the mutations present in NB.1.8.1 may improve its binding to human cells, potentially heightening its transmissibility and giving it a chance for greater dominance over other existing strains.
Although the indications are strong that NB.1.8.1 may be more transmissible, the full scope of how it stacks up against other circulating strains is still uncertain. Fortunately, there is no conclusive evidence suggesting that Nimbus leads to more severe illness compared to other variants, and WHO considers the global risk posed by NB.1.8.1 to be ‘low.’
Despite this, the trajectory of NB.1.8.1’s case counts remains uncertain. Experts express curiosity about whether this new variant will surpass current dominant strains or instigate a summer surge in cases across the U.S.
Health experts caution that predicting the behavior of a new variant is challenging. Dr. Albert Ko, a professor at Yale School of Public Health, remarked, “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.”
High population immunity, with approximately 90% of Americans having been exposed to COVID-19 through infection or vaccination, plays a crucial role in controlling the impact of newer variants like Nimbus. However, the waning of immunity over time and the continued emergence of mutated variants can leave more individuals susceptible to infection.
Andy Pekosz, Ph.D., a professor at the Johns Hopkins Bloomberg School of Public Health, previously noted, “The longer we get from the last surge or the last vaccination campaign, the more people are susceptible.” The combination of new variants like NB.1.8.1, which may possess immune evasion capabilities, the summer travel season, and increased social interactions could facilitate the rapid spread of the virus.
Furthermore, COVID-19 can exhibit surges at various points throughout the year, as indicated by CDC data. In the U.S., cases typically escalate twice: in winter and again in summer, typically between June and August. However, the timing and intensity of these summer surges can differ significantly.
Based on historical trends, experts anticipate a possible increase in COVID-19 cases in the coming months; however, the precise nature of this year’s patterns remains yet to be determined. “We may see an upswing this summer as in previous years, and this is expected to be smaller compared to what we experience in the winter,” Dr. Ko commented.
The symptoms associated with the new variant, NB.1.8.1, largely mirror those seen with recent strains of omicron. These symptoms include sore throat, cough, runny nose or congestion, shortness of breath, fever or chills, headache, body aches, fatigue, and a new loss of sense of smell or taste.
Although anyone can contract COVID-19, certain groups remain at heightened risk for severe illness, including individuals over the age of 65 and those who are immunocompromised or have preexisting medical conditions, according to the CDC.
With respect to vaccination, the COVID-19 vaccine continues to provide substantial protection against severe illness, hospitalization, and death. Amid evolving vaccination guidelines, many may question their eligibility for additional shots. The CDC recommends that the 2024-2025 COVID-19 vaccines are available and suggested for adults aged 18 and older.
Dr. Ko stated, “Those in high-risk groups should consider getting the vaccine now if they haven’t been vaccinated in the last six months and consider receiving an updated vaccine for the upcoming winter season.” Current COVID vaccines remain predicted to be effective against the NB.1.8.1 variant, according to the WHO.
The new 2025-2026 COVID vaccines, which will target the LP.8.1 variant, are anticipated to be released this fall. However, new guidelines announced by Health and Human Services Secretary Robert F. Kennedy Jr. may complicate access for certain groups. The forthcoming vaccines will be limited to adults aged 65 and older and individuals aged 12-64 with at least one underlying condition.
Moreover, the CDC has ceased recommending routine COVID vaccination for healthy children and pregnant women, although many professionals in obstetrics and pediatrics continue to advocate differently.
For those with concerns regarding the COVID-19 vaccine, it’s best to consult with your healthcare provider. Key measures to protect yourself and others from COVID-19 include testing when symptoms present, isolating during illness, wearing masks indoors, and maintaining social distancing.
image source from：https://www.today.com/health/coronavirus/new-covid-variant-nb181-nimbus-symptoms-rcna212304
