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(NEXSTAR) — Where did omicron come from? It’s a question scientists worldwide are still working to understand as the COVID-19 variant continues its reign as the dominant strain globally. While much is known about its symptoms and differences to other variants, its origins remain unclear.

Researchers say omicron is so different from previous strains, like delta, that they’re unsure how it even came to be, Nature scholarly journal reports. In an analysis still awaiting peer review, authors explain omicron’s closest estimated relative was from mid-to-late 2020 – as if this particular SARS-CoV-2 strain developed completely independently of previously widespread ones.

What’s more: omicron has over 50 mutations, more than any other strain. Many of these particular mutations, authors of the analysis say, are very rare or altogether never-before-seen. Some researchers say that simple person-to-person spread would not result in so much viral change. Could it be rodent-human transmission?

A December study of 45 omicron mutations published in the Journal of Genetics and Genomics found the types of changes in those cells resembled those found when coronaviruses previously evolved in mice, researchers said. Single-nucleotide substitutions for RNA viruses in humans typically flip from G to U – but omicron shows a switch from C to A.

Some scientists theorize that coronavirus may have been transmitted from a person to a rat and then back to a person, accounting for some of omicron’s unique mutations, which Scripps Research infectious disease researcher Kristian Andersen says have rarely been seen in humans.

Rats, mice and disease

Rodents are especially good at spreading disease because they live and/or frequent places where germs accumulate, Dr. Bobby Corrigan, urban rodentologist, tells Prevention. Mice and rats live and visit trash bins and sewers, touching everything with little hands that never get washed.

Rats and mice can transfer dozens of diseases to humans in two ways: directly or indirectly. Direct exposure happens when a human comes into contact with rodent feces, urine or even the creatures themselves. This happens easily, since Corrigan says rats and mice are “constantly urinating and defecating” inside human homes.

Indirect transmission happens when rodent parasites – like ticks or fleas – infect humans with various diseases, including West Nile virus and Lyme disease.

Omicron symptoms include cold or flu-like symptoms, including runny nose, fatigue, sneezing and sore throat – night sweats, itchy eyes, nausea and vomiting have also been reported.