Top US scientist Anthony Fauci said Tuesday that while it would take weeks to judge the severity of the new Covid-19 variant Omicron, early indications suggested it was not worse than prior strains, and possibly milder.

Speaking to AFP, President Joe Biden's chief medical advisor broke down the knowns and unknowns about Omicron into three major areas: transmissibility, how well it evades immunity from prior infection and vaccines, and severity of illness.

The new variant is "clearly highly transmissible," very likely more so than Delta, the current dominant global strain, Fauci said. 

Accumulating epidemiological data from around the world also indicates re-infections are higher with Omicron. 

Fauci, the long-time director of the National Institute of Allergies and Infectious Diseases (NIAID), said lab experiments that tested the potency of antibodies from current vaccines against Omicron should come in the "next few days to a week."

On the question of severity, "it almost certainly is not more severe than Delta," said Fauci.

"There is some suggestion that it might even be less severe, because when you look at some of the cohorts that are being followed in South Africa, the ratio between the number of infections and the number of hospitalizations seems to be less than with Delta."

But he added it was important to not over-interpret this data because the populations being followed skewed young, and were less likely to become hospitalized.

"I think that's going to take another couple of weeks at least in South Africa," where the variant was first reported in November, he said.

"As we get more infections throughout the rest of the world, it might take longer to see what's the level of severity."

Fauci said a more transmissible virus that doesn't cause more severe illness and doesn't lead to a surge of hospitalizations and deaths was the "best case scenario."

"The worst case scenario is that it is not only highly transmissible, but it also causes severe disease and then you have another wave of infections that are not necessarily blunted by the vaccine or by people's prior infections," he added.

"I don't think that worst case scenario is going to come about, but you never know."