The U.S. Centers for Disease Control–affiliated lab Helix, based in San Mateo, Calif., found two unique cases of Delta-Omicron hybrids when sequencing nearly 30,000 positive COVID samples obtained from U.S. individuals between November and February, according to the study, published to a preprint server for health sciences papers that haven’t yet been peer reviewed, cofounded by Yale University and The British Medical Journal.

The Delta-Omicron hybrids—SARS-CoV-2 genomes with features of both Delta and Omicron variants of COVID, known as recombinants—are rare, according to the study, which added that there is no evidence such mutations spread more easily than the highly transmissible Omicron.

Additionally, the study identified 20 cases in which individuals were infected with both Delta and Omicron at the same time, including one such case that included a low level of recombinant virus. Two cases of coinfection have already been reported in another study currently under review, the study’s authors stated.

Dr. Leondios Kostrikis, a virologist at the University of Cyprus, announced Jan. 7 that he had identified several so-called Deltacron recombinants. His team uploaded 25 of the sequences to GISAID, an international research organization that tracks changes in COVID and the flu virus, that day, and 27 more a few days later, according to a Jan. 21 Nature piece titled “Deltacron: The Story of the Variant That Wasn’t.”

The next day, Bloomberg picked up the news. Overnight, Deltacron became an international story.

Not so fast, some experts cautioned, with many insisting that a recombinant hadn’t been born, but that the sequences discovered were the likely product of laboratory contamination.

But the naysayers were wrong. World Health Organization COVID-19 technical lead Dr. Maria Van Kerkhove, an infectious disease epidemiologist, addressed the variant at a Wednesday media briefing, acknowledging the existence of the blend of Delta, also known as AY.4, and Omicron, also known as BA.1.

It had been identified in France, the Netherlands, and Denmark, she said, adding that levels of detection were “very low” and that such mutations come as no surprise.

“This is something that is to be expected, given the large amount of circulation, the intense amount of circulation we saw with both Omicron and Delta,” she said.

“This is what viruses do. They change over time.”

Additionally, COVID is infecting animals, with possibility of infecting humans again, creating additional chances for mutations.

“So, again, this pandemic is far from over,” she said. “We cannot allow this virus to spread at such an intense level.”

The development of recombinants is common among viruses, said Dr. Phoebe Lostroh, a Harvard-trained microbiology professor at Colorado College, a private liberal arts college in Colorado Springs.

Microbes—which include bacteria, viruses, fungi, and protozoa—”evolve faster than we do because they can reproduce in such a short time,” she said.

Case in point: the flu.

“The interesting thing is that every global [flu] pandemic since 1918 has had at least some genes from the flu pandemic of 1918–1919, all these years later,” she said.

The Helix study’s findings are highly unlikely to have been caused by contamination or technical artifacts, the authors asserted, citing resequencing and the processing of coinfected samples on different days, among other evidence.

A study published Tuesday in The Journal of Clinical Investigation found evidence of five fully vaccinated Spanish COVID patients with breakthrough infections of the Alpha variant, which swept the United Kingdom in late 2020, with features of the Delta Plus, Iota, and Omicron variants.

“Deltacron” is a media portmanteau. Scientists and have not yet officially named the variant. Neither the World Health Organization nor the CDC has named it a variant of concern.