Little is known yet about the highly-mutated Omicron variant but experts have already speculated about how it came to exist.
Professor Edward Holmes, who is known for his work on the emergence of infectious diseases, has shared the main theories among his colleagues with the ABC’s medical commentator, Dr Norman Swan.
Omicron could have emerged in September
Some believe the new variant of concern has been circulating for several months despite the first case being detected in South Africa in early November.
“Some evolutionary biologists have suggested it may have emerged in September,” he told the ABC’s Coronacast podcast.
Kristian Andersen, an infectious disease researcher at Scripps Research in San Diego, also estimates the virus may have emerged around late September or early October, which suggests it may be spreading slower than it appears to have, according to scientific journal Science.
Covid variant may have evolved in immunocompromised
Another line of thought is that it came from a person who is severely immunocompromised.
Leading South African virologist Professor Alex Segel has published a new paper which outlines a case study of a person with HIV who had a Covid infection for several months.
“Huge amounts of virus being produced, and in fact produced a mutated variant which while not identical to Omicron, it had some evolutionary similarities,” Dr Swan explained.
The study found this particular strain was vaccine resistant.
“Because they have a weakened immune system, they have a weaker antibody response and it gives the virus a chance to mutate around the antibodies and resist them,” he added.
“Whereas a stronger antibody response would knock off the virus and not give the virus a chance to evolve.”
Despite the waning effectiveness of the vaccine, the immunocompromised patient stayed well and didn’t show many symptoms.
“So this is a person who is immunocompromised that you would expect to get battered by Covid-19 but they weren’t. So it was a mild disease in this person," Dr Swan said.
“There are big differences between that virus and Omicron, but it does fit with some of the things you’re hearing about Omicron".
Omicron possibly came from an animal
The third but most unlikely theory is that the virus went back into an animal before reinfecting a human, which could explain the large number of mutations in the Omicron strain.
Dr Swan said the Omicron strain did not appear to have descended from the deadly Delta strain, but rather the B.1.1 lineage, similar to the Alpha variant in the UK.
Fears new strain 'more dangerous'
Early evidence also suggests it resembles the Beta strain, which was previously the most vaccine resistant type of Covid-19, but not contagious enough to overtake the prevalent Delta variant.
“It’s got elements of Beta in it but it has a lot more in addition to Beta which makes it potentially more dangerous," Dr Swan said.
Scientists are still working to determine if Omicron is more transmissible, more at risk of evading diseases or leads to more severe diseases.
The World Health Organisation has warned it poses a “very high” global risk and could lead to surges with “severe consequences”.
Do you have a story tip? Email: email@example.com.