WATCH: Dr. Fauci says world has been ‘shocked’ by omicron spread

America’s leading infectious disease expert Dr. Anthony Fauci said Monday the world had been “shocked” by the emergence of the omicron variant of the coronavirus as more countries are reporting surges in infections.

Watch Fauci’s remarks in the player above.

Speaking at a National Press Club event, Fauci said omicron appeared “even more transmissible than the very troublesome delta variant” which had become the most common worldwide.

“It is now dominating in Africa, has taken over in England. And right now, as I speak to you in the United States, more and more of the isolates that are being identified in sequence are of omicron,” Fauci said.

Omicron accounted for 73 percent of new infections last week, a nearly sixfold increase in only one week, U.S. federal health officials announced.

Fauci said it has raised alarm because of its sheer number of mutations, more than prior variants had.

Possibly 30 are in the key spike protein that lets the virus attach to human cells.

Many critical questions about omicron remain unanswered, including whether the virus causes milder or more severe illness and how much it might evade immunity from past COVID-19 illness or vaccines.

Fauci believes the variant is “unlikely” to be more severe, but it could “possibly” be less so.

Cases are surging in parts of the U.S., particularly the Northeast and Midwest, though it’s not always clear which variant is driving the upswing.

In New York City, where the mayor has said the new omicron variant is already in “full force,” a spike is scuttling Broadway shows and spurring long lines at testing centers, but so far new hospitalizations and deaths are averaging well below their spring 2020 peak.

Pfizer said Friday it was changing plans and testing three doses of its COVID-19 vaccine in babies and preschoolers after the usual two shots didn’t appear strong enough for some of the children.

Pfizer announced the change after a preliminary analysis found 2- to 4-year-olds didn’t have as strong an immune response as expected to the very low-dose shots the company is testing in the youngest children.

It’s disappointing news for families anxious to vaccinate their tots.

Pfizer had expected data on how well the vaccines were working in children under 5 by year’s end, and it’s not clear how long the change will delay a final answer.

Pfizer and its partner BioNTech said if the three-dose study is successful, they plan to apply for emergency authorization sometime in the first half of 2022.

“We already know that this is going to be effective, but you’ve got to prove it, that it’s safe and effective at a certain dose,” Fauci said.