Explainer: How worried should we be about the Covid-19 Omicron variant?

The Omicron coronavirus variant has been detected in 77 countries since it was first identified three weeks ago, fuelling concerns that its large number of mutations will help it spread faster and evade protection provided by Covid-19 vaccines or prior infection.

Scientists await answers to these important questions:

Is it more contagious?

The fast-spreading Delta variant remains dominant worldwide, and it is unclear whether Omicron is inherently more contagious than its predecessor, the World Health Organization (WHO) said in a brief issued on Sunday.

Early data suggests that the new variant is spreading faster than previous versions of the virus. In South Africa, the UK and Denmark, the number of new Omicron infections has been doubling every two days - "an alarming rate of growth," according to Dr Eric Topol, director of the Scripps Research Translational Institute in La Jolla, California.

The Omicron variant accounted for about 44 per cent of infections in London on Monday and was expected to become the dominant version of the virus there within 48 hours.

What type of illness does it cause?

Scientists say it is still too early to know whether Omicron causes more or less severe Covid-19 than previous versions of the virus.

In South Africa, scientists have said they see no sign that the Omicron variant is causing more severe illness. Hospital data found that Covid-19 admissions were rising sharply in more than half of the country's nine provinces, but there have been comparatively few deaths and indicators such as the median length of hospital stay have been reassuring.

Among the 43 people identified as having the Omicron variant in the United States, most reported mild symptoms including coughing, congestion and fatigue.

People so far infected with Omicron have largely reported mild illness likely because many have at least some immunity from vaccinations and/or prior infection.

A more clear picture of Omicron's severity will come from analysing outcomes for a larger number of infected people, particularly older, unvaccinated, previously uninfected patients.

Can it re-infect people or evade vaccine protection?

The Omicron variant has about 50 mutations not seen in combination before, including more than 30 mutations on the spike protein the coronavirus uses to attach to human cells. Most current Covid-19 vaccines target that spike protein, making the cluster of mutations potentially problematic.

Experimental antiviral pills - such as Pfizer Inc's Paxlovid and Merck & Co Inc's molnupiravir - target parts of the virus that are not changed in Omicron. These drugs are likely to become important weapons if vaccine-induced and natural immunity are threatened.