Here are the top 10 points in this big story:
Data at this time shows Omicron was in circulation well before it was flagged in South Africa last week. The fact cases were reported in Hong Kong, Israel and Belgium within 72 hours of the variant being identified underlines the (unwelcome) news that there may be many more Omicron cases in the world.
Since its detection, the Omicron strain has been reported in a dozen countries, including the United Kingdom and mainland Europe, and Japan. This week Germany confirmed its first cases - travellers from South Africa - and Italy its first - a passenger from Mozambique. The strain may also be present in the Netherlands, where 61 arrivals on two flights from South Africa have tested positive for COVID-19.
The United States, India and China - the three countries worst affected by the entire pandemic - have not yet reported Omicron cases. International airports across India have begun stringent screening and testing of incoming passengers, particularly those from 'at risk' countries. A senior doctor at the government-run ICMR, however, told NDTV it is likely Omicron is already present in the country.
Researchers are still working on establishing if Omicron - which seems to be able to evade vaccines better than any other strain - is more lethal and, crucially, if current vaccines offer any protection at all. They are also checking to see if Omicron is more transmissible than Delta, which is already highly infectious. The WHO has suggested this is the case; it said Omicron poses a "very high" global risk.
If Omicron does, in fact, prove to be even more transmissible than Delta, it could lead to a sharp spike in infections that could put pressure on hospitals. And that is bad news for India, where existing medical infrastructure is poor and was barely able to contain damage caused by the Delta variant.
It might not be all doom-and-gloom, though, with anecdotal evidence from doctors in South Africa suggesting Omicron may be more infectious but produces milder symptoms than Delta. Adrian Puren, acting head of South Africa's National Institute for Communicable Diseases, told Reuters it could take four weeks for experts to arrive at a more definitive understanding of vaccine evasion and symptoms.
According to a report by Reuters, most current PCR tests cannot distinguish Omicron from the Delta variant, the dominant and most infectious version of the virus so far. To distinguish between them, the test must identify an Omicron mutation - the S-gene drop-out or S-gene target failure (SGTF). However, this is not a definitive criterion because the Alpha variant also has that mutation. The understanding, though, is that because Alpha is not widely circulating, the presence of the SGTF indicates Omicron.
Existing Covid tests (PCR and rapid), therefore, will still detect Covid infections - as the Indian government told states and UTs this week - but it cannot definitively identify the variant. The WHO has recommended the US-made TaqPath test kit, but it is unclear how many countries have invested in this.
The speed at which virologists and other scientists are working to track, sequence and understand the Omicron strain is in stark contrast to the response to Delta and other variants. The WHO, for example, took months to designate Alpha a 'variant of concern' when it emerged in June last year, but only days to identify Omicron as a global threat.
The discovery of the Omicron variant and the initial flurry of travel bans - the UK, the US and other major economies were among the first to suspend flights to and from southern Africa - led to turmoil in global financial and oil markets amid uncertainty over the strain's capabilities and potential impact.
With input from AFP, Reuters