Certain features of the Omicron variant, including its global spread and a large number of mutations, suggest it could have a major impact on the course of the COVID-19 pandemic, said the chief of the World Health Organization (WHO).

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With the Omicron variant now present in 57 countries, WHO Director-General Tedros Adhanom Ghebreyesus warned at a press briefing that it can spread more rapidly than previous variants, Xinhua news agency reported. "We are now starting to see a consistent picture of the rapid increase in transmission (rates), although for now the exact rate of increase relative to other variants remains difficult to quantify. Emerging data from South Africa suggest increased risk of re-infection with Omicron, but more data are needed to draw firmer conclusions," he added.

While some evidence might suggest that Omicron causes milder symptoms than the earlier Delta variant, it's still early days to draw any final conclusions, WHO experts have said. According to Mike Ryan, executive director of the WHO's Health Emergencies Program, although the evolutionary nature of the virus makes it more transmissible as it mutates, this doesn't necessarily make the virus less severe, as has been suggested by some "urban legends."

Whether or not a mutation turns out to be milder or more lethal is a matter of chance, he said. As studies of the latest COVID-19 variant are evolving, the WHO says it still needs days or even weeks for global epidemiological data to come in, be analysed and then to draw any firm conclusions.

It's also still premature to say that Omicron could result in a significant reduction in vaccine effectiveness, according to WHO Chief Scientist Soumya Swaminathan. The WHO has called on all countries to increase surveillance, testing and sequencing, and to submit more data to the WHO Clinical Data Platform using an updated online case reporting form.