Covid, Italian variant hunter: "This is how I intercepted Nimbus"

It gave its first sign on January 22, 2025. In fact, according to the World Health Organization, the first sample collected dates back to that date. Since then, the new variant of Sars-CoV-2 that has become under special observation, NB.1.8.1, already dubbed ' Nimbus' by experts on social media, has grown in silence . The first to notice it was the Italian variant hunter, Federico Gueli . In 2025, he told Adnkronos Salute , "China had uploaded very few sequences". Then at a certain point the numbers began to rise. "And when a country starts to upload more sequences, we increase the level of attention. Now, with the reduction in the activity of sending sequences, we can examine them one by one and it's easier. With routine work that we do daily, we monitor the lineages extensively". At the time of the Covid boom "we even got to monitor 450 at the same time. We check if they grow, what mutations and what advantages they have".

In this case, the progenitor of the Nimbus variant was already dominant in China "and I - continues Gueli's reconstruction - noticed 3 or 4 sequences with mutations , one of these had already emerged and was considered advantageous in other lineages. I therefore proposed it to the Pango system", an open access platform that tracks what has now become an enormous tree of sequences full of ramifications that photographs the evolution of Covid.

Returning to the new entry NB.1.8.1 today it is classified as a variant under monitoring by the WHO . The UN agency's report explains that it is a descendant lineage of XDV.1.5.1, in turn a descendant of JN.1. Compared to the currently dominant Sars-CoV-2 variant, i.e. LP.8.1, Nimbus has additional spike mutations in a position where it is believed they can increase transmissibility. Up to May 18, there were 518 sequences of NB.1.8.1 sent to the Gisaid database from 22 countries (but today "there are already more than a thousand", Gueli specifies), 10.7% of the sequences available globally in epidemiological week 17 of 2025, which goes from April 21 to 27. Although the percentage remains low - WHO warned - it is a significant increase compared to the 2.5% recorded 4 weeks earlier (from March 31 to April 6, 2025). This variant has also been detected in greater circulation in the European region, from 1% to 6%.

The Nimbus variant "certainly has an origin in China", explains Gueli. It is the result of "multiple recombinants that have recombined in turn over a long period of time and have continued to evolve". The nickname chosen for it "refers to the cloud and was chosen because it recalls Nb in the name", he says. "But it is not the only variant that we are keeping an eye on". "The scientific studies that we have at the moment, conducted promptly also on NB.1.8.1" by the team of the Chinese scientist "Yunlong Cao, also thanks to the work of the community" of variant hunters, "have highlighted another variant of interest, another recombinant, XFG. All the variants that we are seeing at the moment are recombinants. Probably the virus at this stage is no longer able to obtain an advantage with evolution alone and it is better for it to mix parts of the genome to put together the most advantageous mutations, a mix of greater evasive capacity and better transmissibility".

Gueli then explains the story of an Indian variant. " This country is not sequencing much at the moment , but we also look at the sequences in American airports, the samples sequenced from travelers entering America. In some there is an indication of the country of origin. In about 50% of the sequences from this country a new variant is detected. Although on a few sequences it is indicative up to a certain point, it is a sign that the prevalence in India is quite high. Right in the same period, we also notice that there are journalistic reports from the country in question that indicate an increase in cases in a specific area. This variant seems quite fast and would be a third type of evolution, it has in the spike mutations found for the first time in Africa, which have recombined several times, giving rise to different" lineages. "The Indian lineage - says Gueli - has just been designated XFP". And the work of the variant hunters continues.