On June 25, 2025, the World Health Organization’s Technical Advisory Group on Virus Evolution published risk evaluations for two Omicron descendants, NB.1.8.1, nicknamed Nimbus, and XFG, known as Stratus. Both were listed as variants under monitoring. By the end of the year, genomic testing showed the pair had become the most common versions of the virus behind COVID-19.
The WHO rated the global risk from each as low. For both, it expected the COVID-19 vaccines now approved to keep protecting against symptomatic and severe disease.
Nimbus and Stratus
Nimbus descends from a recombinant called XDV.1.5.1. Its spike protein carries mutations that bind more tightly to the ACE2 receptor, which could make it spread more easily. Its share of cases was rising as an earlier variant, LP.8.1, faded.
Stratus is a recombinant of two lineages, LF.7 and LP.8.1.2, and was one of seven variants under monitoring at the time. Several countries in Southeast Asia saw new cases and hospitalizations climb together where it circulated. Even so, the WHO found that infection with Stratus did not produce worse illness than other circulating strains.
Later work from Guo and colleagues, published in The Lancet Infectious Diseases, found that Stratus evades immunity but binds ACE2 relatively weakly. Nimbus showed a more balanced profile of the two traits, which the researchers said supports its chances of becoming more common.
Where they dominate
Stratus leads in Europe and the Americas. Nimbus leads in the Western Pacific region. From mid-2025, reported COVID-19 cases rose week over week, mainly in North America and Europe. By November 2, 2025, the WHO had received reports of 778,900,250 cases since 2020.
That count is almost certainly low. Testing and reporting fell after the pandemic ended, and surveillance now leans mainly on hospital admissions. The WHO and the UK Health Security Agency have both raised concerns about complacency.
A December 2025 editorial in Medical Science Monitor argued that this complacency has fed a worrying rise in infections from the endemic Omicron variants. Its author wrote that variants of SARS-CoV-2 “must now be increasingly considered contenders for not-too-distant future pandemics.”
How the tracking system got here
The WHO declared COVID-19 a public health emergency of international concern on January 30, 2020. By March 2020 it was calling the outbreak a pandemic, with more than 118,000 reported cases in 143 countries and 4,291 deaths. On May 31, 2021, its Virus Evolution Working Group revised how variants of concern and variants of interest are named. Omicron joined the variant of concern list on November 26, 2021. The WHO’s Director-General announced the end of the pandemic on May 5, 2023, and COVID-19 was classed as an ongoing, established health issue.
Another Omicron lineage is on researchers’ radar. BA.3.2 was detected in South Africa in November 2024 and carries more than 50 mutations compared with its ancestor, BA.3. It evades antibodies but binds ACE2 poorly and is not very infectious so far. Guo and colleagues wrote that it could become a dominant Omicron variant in the coming year.
