Top US scientist Anthony Fauci said Tuesday early indications suggested the Covid-19 Omicron variant was not worse than prior strains, and was possibly milder, while cautioning it would take weeks to judge its severity.
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 results from 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 noted it was important to not over-interpret this data because the populations being followed skewed young and were less likely to become hospitalized. Severe disease can also take weeks to develop.
"I think that's going to take another couple of weeks at least" to confirm in South Africa, where the variant was first reported in November, he said.
"Then 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."
The Omicron variant has now been detected in at least 38 countries.
Though it has not yet been linked to any deaths, scientists are particularly concerned by the unique "constellation" of more than 30 mutations on the spike protein that dots the surface of the coronavirus and allows it to invade cells.
Fauci said the science remains unclear on how the variant originated, but there are two main theories.
Either it evolved inside the body of an immunocompromised patient, such as a person with HIV who failed to rapidly fight off the virus.
Or, the virus could have crossed from humans to animals, then returned to people in a more mutated form, in an example of "reverse zoonosis."
Asked whether vaccinated people should act more cautiously given the unknowns, Fauci said the public should remain prudent, particularly during travel, and wear a mask when gathering indoors where the vaccination status of others is unknown.
Those who are fully vaccinated should also seek a booster when eligible, he stressed.
Booster shots have been shown to drastically increase the level of antibodies that bind to the spike and also translate to better disease outcomes in the real world, as seen in Israel, which embarked on its booster campaign earlier than the United States, said Fauci.
But, while boosters heighten the intensity and breadth of a person's immune response, it's still too soon to know how durable the response will be and whether additional shots might be required in future, he added.
On Tuesday, the United States hit its latest vaccination milestone, with 60 percent of the population fully vaccinated -- but Fauci said there remained a long way to go.
The 80-year-old physician and scientist stressed that misinformation continued to hamper the country's pandemic response, with the problem particularly acute in Republican-led states.
"Misinformation is still a really, really important detriment to an adequate response," he said.
"We have in this country still 60 million people who are eligible to be vaccinated and have not gotten vaccinated, and much of that is along ideological and party lines, which is very unfortunate."
So-called "red states" led by Republicans continue to be less vaccinated than "blue" states run by Democrats.
"There's no reason for that, that should not be, we should have a uniform public health attitude," he concluded.