Stratus (XFG) Sweeps In, Nimbus Fades, Leaving COVID-19 Boosters Unreliable

Rapid Mutation of Dominant Strains Vexing Bio-Pharmaceutical Complex

By Peter A. McCullough, MD, MPH

The annual COVID-19 booster campaign kicked off a few months ago with news stories about Nimbus or “razor-blade sore-throat” effectively marketing for the new shots with hope that NB.1.8.1 would be covered by the monovalent 2025 COVID-19 boosters coding for the JN.1-lineage, specifically the LP.8.1 strain. This formula was chosen by the FDA to target the versions of the virus that were most common earlier in 2025.

On cue, SARS-CoV-2 has rapidly mutated to the XFG strain. The XFG strain, also known as “Stratus,” is a recombinant variant within the Omicron family. It is a hybrid of two other Omicron sub variants: LF.7 and LP.8.1.2.

This means that XFG inherited genetic material from both parent strains, which can give it new properties, such as being more transmissible or better able to evade vaccine immunity. This recombination process is one of the ways the SARS-CoV-2 virus continues to evolve to outsmart the vaccine developers.

There is no clinical trial evidence that the 2025 COVID-19 booster, also known as the 2025–2026 formula, provides protection against Stratus (XFG). The primary hope for the booster is that vaccine antibodies targeting JN.1 lineage that would also neutralize Stratus. LF.7 is a subvariant is also a descendant of the JN.1 subvariant. LF.7 was previously detected in several countries, including India, Singapore, and Hong Kong, where it contributed to surges in cases in mid-2025.

USA Today reported on the conundrum:

Pfizer said pre-clinical data show its vaccine protects against multiple circulating variants, including XFG (Stratus), NB.1.8.1 (Nimbus) and other emerging strains. Novavax said studies confirm its shot induces immunity against JN.1 lineage strains.

The updated COVID vaccines should provide protection that lasts up to four to six months, Dr. Georges Benjamin, executive director of the American Public Health Association, tells TODAY.com.

So people going out for boosters should know that the current JN1 booster: 1) has not been tested in large RCT’s for safety and efficacy, 2) only lasts a few months, 3) has no published human data whatsoever with the current Stratus strain.

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Peter A. McCullough, MD, MPH

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