COVID Update: COVID Cases Rising Worldwide: Inside the NB.1.8.1 (Nimbus) vs. XFG (Stratus) Variant Battle
New COVID Surge Explained: A Deep Dive into NB.1.8.1 (Nimbus) vs. XFG (Stratus)
In this post, we take a deeper look at what sets NB.1.8.1 apart—its unique symptom profile, its virological advantages, and why it's spreading so rapidly. We’ll also compare it head-to-head with the emerging XFG lineage (including XFG.3 and XFG.4), which is gaining traction around the world. XFG has shown even greater immune escape in laboratory studies. You’ll learn how these variants differ in their spike protein mutations, how they interact with population immunity, and what this means for vaccine protection, reinfection risks, and public health moving forward.
For more than five years, TACT has consistently been ahead of the curve in identifying not just which variants are likely to drive new COVID waves, but also when those waves will occur. The rapid rise of NB.1.8.1—nicknamed Nimbus—is yet another example. In our April 14th update , we predicted that NB.1.8.1 would become the next dominant variant. That forecast has now proven accurate: according to the latest CDC variant data, NB.1.8.1 makes up over 43% of all sequenced cases in the United States. Back in early April, few, if any COVID trackers were saying NB.1.8.1 would be the next dominant variant. The World Health Organization didn’t designate it as a Variant Under Monitoring (VUM) until May 23, 2025, well over a month after TACT raised the alarm.
This delay is part of a broader pattern. The WHO has consistently lagged in recognizing both transmission trends and the long-term health risks posed by SARS-CoV-2—including persistent infection and post-acute sequelae, which remain largely unaddressed by global public health agencies. Meanwhile, a pre-print published on June 17, 2025 confirmed what TACT warned months earlier: NB.1.8.1 has a significant growth advantage over other circulating variants and is on track to become globally dominant.
CDC Variant Proportions - Nowcast
According to the CDC’s Nowcast data, XFG is also gaining traction, though its ascent has been slower than NB.1.8.1’s. In April, XFG lacked the spike mutations needed to confer a significant growth advantage—but that’s changed. As the lineage continues to evolve, XFG.3 and XFG.4 are now emerging as a serious contenders in the ongoing variant race. XFG.4, with the greater growth advantage, has already been sequenced in Asia, India, Australia, New Zealand, the Middle East, across Europe, Canada, the U.K. and the United States.
What’s deeply concerning, however, is the CDC’s response. Rather than expanding genomic sequencing to keep pace with these changes, the agency has opted to delay public reporting of variant proportions beyond the current two-week lag, citing the lack of sequencing which led to the delay for more accurate Nowcast modeling. In practice, this decision further erodes our already diminished capacity to track emerging threats in real time—a dangerous move at a moment of accelerating viral evolution.
The authors of the June 17 preprint captured the stakes with chilling precision:
“As NB.1.8.1 is a product of intricate evolutionary events, the spreading of this variant would further complicate the prediction about future evolution and spreading trend of the SARS-CoV-2, highlighting the importance and necessity of performing genomic surveillance and monitoring SARS-CoV-2 evolution.”
That call for vigilance couldn’t be more timely—and yet, public health agencies seem to be stepping back, just when they should be stepping up.
📈 Wastewater Signals Point to Rising Prevalence in the South and West
As expected, COVID activity is rising fastest in the Southern U.S., followed by the West, according to the latest wastewater surveillance data as of June 23rd. In contrast, the Northeast and Midwest are trending downward—but not for long. Based on regional patterns and past wave timing, increases in those areas are likely within the next 2 to 4 weeks.
🧠 Why Is the South Surging While the Northeast and Midwest Dip?
Here’s a question for the informed: Why are southern states seeing rising prevalence while the Northeast and Midwest are still showing a slight decline?
If you’ve been following TACT for a while, you probably already know the answer. Let’s put it to the test:
Walgreens Respiratory Index
A tool we have been utilizing for years now is the Walgreens Respiratory Index. Although the number of tests being conducted has diminished quite a bit, the positivity rate is a great early indicator of which way cases are trending. As we can see below, the positivity rate has been increasing, particulary across many of the southern states. This isn’t as helpful as in the past due to who’s getting tested and the number of people testing, but it still worth noting since it is in line with the wastewater testing.
🧬 NB.1.8.1 (Nimbus) Symptom Snapshot: Razor Throats, “Weird” Aches, and Prolonged Fatigue
As NB.1.8.1—nicknamed Nimbus—continues its rapid global ascent, many are reporting symptom patterns that feel noticeably different from earlier Omicron waves. Though the World Health Organization currently classifies it only as a Variant Under Monitoring, its real-world clinical footprint is anything but ordinary.
😖 “Razor Blade” Throat: The Defining Symptom
One of the most striking—and widely reported—symptoms is a sudden, severe sore throat often described as feeling like swallowing razor blades or broken glass. Unlike typical COVID sore throats, this can appear abruptly and become excruciating within 24–48 hours. Some compare it to the worst strep they’ve ever had, and for a subset, it’s the only obvious sign of infection.
🧠 Oddly Placed Muscle Aches
Another standout feature is muscle pain in unusual places—behind the ears, in the jaw, upper back, neck, or shoulders. These aren't the generalized aches familiar from past waves, but more localized discomfort that suggests a shift in tissue inflammation patterns, possibly linked to NB.1.8.1’s unique spike mutations and how they interact with mucosal and neural tissues.
🛑 Persistent Congestion and Low-Energy Crash
Nasal congestion is also sticking around longer than usual—often lasting more than a week, and less responsive to over-the-counter meds. That could mean a longer window of infectiousness, which would help explain this variant’s rapid spread. Alongside it, many report a fatigue that lingers well after other symptoms subside.
📊 Compared to Other Variants
NB.1.8.1 doesn’t appear to increase hospitalization risk on a per-case basis, but its distinctive symptom profile includes:
Sharp, sudden sore throat
Muscle aches in unusual locations
Longer congestion duration
Fatigue and brain fog out of proportion to mild fever or cough
In short, the volume is turned up in all the wrong places and many of these signals point towards a greater risk of persistent infections. Unfortunately, we won’t find out if there is a greater risk of persistent infections for many months, if not years.
🔎 What’s Behind These Symptoms?
The most likely explanation is NB.1.8.1’s constellation of spike protein mutations, which may improve its ability to infect upper airway tissues, particularly the throat. There's also growing suspicion that variant-specific immune interaction may be changing how and where inflammation shows up in the body.
🧪 Don’t Dismiss a Sore Throat
Even without fever or a positive rapid test, the combination of a razor-sharp sore throat, localized muscle pain, and stubborn congestion should raise red flags. Isolation, masking, hydration, and rest remain key—especially given how easily this variant is spreading.
🔐 What’s Next: The Summer Variant Race
NB.1.8.1 is now the dominant variant in the U.S.—but it’s not alone. In the subscriber-only section, we’ll dive into the emerging XFG lineage, which is quietly gaining ground around the world. The latest XFG subvariants show a greater growth advantage than the NB.1.8.1 variants and its immune escape profile suggests it's one to watch.
We’ll compare the two variants head-to-head:
Which one spreads faster—and why?
How do their spike mutations differ?
And what do current trends tell us about what to expect this summer in the Northern Hemisphere—and winter in the Southern Hemisphere?
We’ll also reveal the correct answer to the poll above—and why it matters more than ever in understanding wave timing.
👇 Continue reading below for the full breakdown.
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