Will the Omicron variant of SARS-CoV-2, the virus which causes Covid-19, be the black swan that pulls the world back — just when the pandemic seemed to be fading? Global markets certainly seem to think so, with sharp falls on Asian and European trading this morning. But what do we know about the new variant?

The variant — otherwise known as B.1.1.529 — was only identified in Botswana on Tuesday. We have had numerous variants emerge this year, few of which have succeeded in establishing themselves as a real concern. But what is worrying virologists about Omicron is that it has an unusually high number of mutations — 50, according to South Africa’s Centre for Epidemic Response and Innovation. Moreover, 30 of these are in the virus’s spike protein, which it uses to attach itself to human cells. This is concerning because it is the spike protein on which Covid vaccines have been working. If the spike protein changes, it could potentially create a variant that may evade existing vaccines.

But is it more transmissible, and does it cause more severe illness? So far, we have little idea and will not know for weeks while we wait for data to be examined. It took four weeks, for example, after the discovery of the Alpha — or Kent — variant last December before we could start to quantify its additional transmissibility. But we do know that there has been a spike of cases in the Gauteng Province of South Africa — which covers Johannesburg and Pretoria — and that 90 per cent of samples from the past few weeks have been of the Omicron variant. However, that is not a huge amount of cases: 77 so far have been identified in South Africa, plus four in Botswana. And the daily number of new cases in South Africa is still fairly low: 2,465 yesterday.

But has it spread? A case has been identified in Hong Kong and one in Israel, both in people who had travelled from South Africa. But identifying variants is all rather hit and miss — some countries sequence a lot of samples, others do little or none. So the new variant could be present in many countries without us knowing. If it is already in Britain we should soon know about it as we do a lot of sequencing — nearly a quarter of the 5.4 million samples uploaded to the international Gisaid database have come from Britain.

The government has reacted very quickly to this variant — in contrast to April when it waited a few weeks after the emergence of Delta before placing India on the red list. So can we keep it out of Britain? Almost certainly not if Omicron is highly transmissible. Many countries tried to clamp down against the Delta variant by restricting international travel but it made little difference in the end. It is also doubtful whether travel restrictions could buy us much time to re-engineer vaccines. When the original South African variant — Beta — emerged last year, Pfizer suggested it would take several weeks to reprogramme its mRNA vaccine to cope with the new challenge. However, the vaccine would then need to be manufactured, distributed and put into people’s arms — which would take weeks or months. By then, if Omicron really does have the ability to sidestep existing vaccines, it is likely that it will already be endemic.