TOKYO (BLOOMBERG) - The Omicron variant of Covid-19 is 4.2 times more transmissible in its early stage than Delta, according to a study by a Japanese scientist who advises the country's Health Ministry, a finding likely to confirm fears about the new strain's contagiousness.

Dr Hiroshi Nishiura, a professor of health and environmental sciences at Kyoto University who specialises in mathematical modeling of infectious diseases, analysed genome data available through Nov 26 in South Africans in Gauteng province.

"The Omicron variant transmits more, and escapes immunity built naturally and through vaccines more," he said in his findings, which were presented at a meeting of the Health Ministry's advisory panel on Wednesday (Dec 8).

Concerns are swirling globally that Omicron could deal the world a bigger blow than even Delta, and the World Health Organisation (WHO) has cautioned that it could fuel surges with "severe consequences".

But a jump in cases in South Africa in the wake of the variant's emergence has not yet overwhelmed hospitals, leading to some optimism that it may cause only mostly mild illness.

Pfizer and BioNTech also said this week that a booster dose of their vaccines could fortify protection against the strain.

Dr Nishiura's study hasn't been peer-reviewed and published in a scientific journal. The new analysis was conducted using the same method he used in a July study published by the Eurosurveillance medical journal on Delta's predicted dominance ahead of the Tokyo Olympics.

Hundreds of researchers globally are racing to understand the new variant, which is the most differentiated strain yet among the five variants of concern identified by the WHO since the pandemic began.

Cases in South Africa have rapidly increased to as many as nearly 20,000 a day since the country first reported Omicron's discovery two weeks ago.

The number of Covid-19 cases in the nation had remained low in the preceding weeks, despite only 26 per cent of the population being fully vaccinated, according to Bloomberg's Vaccine Tracker.

"The vaccination rate was less than 30 per cent and many people were probably naturally infected," Dr Nishiura said. "We need to pay close attention to future trends to see if the same thing will happen in countries where mRNA vaccines are used at a high rate."