The new omicron coronavirus mutant speeding around the world may bring another wave of chaos, threatening to further stretch hospital workers already struggling with a surge of delta cases and upend holiday plans for the second year in a row.

The White House on Wednesday insisted there was no need for a lockdown because vaccines are widely available and appear to offer protection against the worst consequences of the virus. But even if omicron proves milder on the whole than delta, it may disarm some of the lifesaving tools available and put immune-compromised and elderly people at particular risk as it begins a rapid assault on the United States.

“Our delta surge is ongoing and, in fact, accelerating. And on top of that, we’re going to add an omicron surge,” said Dr. Jacob Lemieux, who monitors variants for a research collaboration led by Harvard Medical School.

“That’s alarming, because our hospitals are already filling up. Staff are fatigued,” leaving limited capacity for a potential crush of COVID-19 cases “from an omicron wave superimposed on a delta surge.”

Most likely, he and other experts said at a news briefing Tuesday, an omicron surge is already under way in the United States, with the latest mutant coronavirus outpacing the nation’s ability to track it.

Based on specimens collected last week, the Centers for Disease Control and Prevention said omicron accounted for about 3% of genetically-sequenced coronaviruses nationally. Percentages vary by region, with the highest — 13% — in the New York/New Jersey area.

But Harvard experts said these are likely underestimates because omicron is moving so fast that surveillance attempts can’t keep up.

Globally, more than 75 countries have reported confirmed cases of omicron. In the United States, 36 states have detected the variant. Meanwhile, delta is surging in many places, with hot spots in New England and the upper Midwest. The five states with the highest two-week rolling average of cases per 100,000 people are New Hampshire, Rhode Island, Michigan, Minnesota and Vermont.

Universities are abruptly closing classrooms during finals week with infections multiplying at a fast rate. Both t he NBA and NHL have had to postpone games, and the NFL had its worst two-day outbreak since the start of the pandemic, with dozens of players infected.

Outside the U.S., the president of the European Union said omicron will become the dominant variant in a month and declared that “once again, this Christmas will be overshadowed by the pandemic.”

Scientists around the world are racing to understand omicron, which has a large number of worrisome mutations in important regions of its genetic structure that could affect how it spreads from person to person. How quickly the number of cases doubles, known as “doubling time,” can give a preview of what the disease burden could be in a few weeks.

CDC Director Rochelle Walensky said Wednesday that early data suggests omicron is more transmissible than delta, with a doubling time of about two days.

In Britain, where omicron cases are doubling every two to three days, the variant is expected to soon replace delta as the dominant strain in the country.

The U.K. on Wednesday recorded the highest number of confirmed new COVID-19 infections since the pandemic began, and England’s chief medical officer warned that the situation is likely to get worse as omicron drives a new wave of illness during the holidays.

“The data out of the UK are quite alarming at this point,” and foreshadow what’s to come in the United States, said Bronwyn MacInnis, director of pathogen genomic surveillance at the Broad Institute of MIT and Harvard. For example, she said, by Tuesday afternoon, omicron was already the most common variant in London.

In many ways, omicron remains a mystery. Hints are emerging from South Africa, where it was first reported, indicating it may cause less severe disease than delta but be better at evading vaccines.

But, MacInnis warned: “There’s much more that we don’t know about this variant than we do, including the severity.”

At the same time, Lemieux said, there seem to be fewer tools to fight it. Some monoclonal antibody treatments don’t work as well against omicron in lab tests, Lemieux said. Vaccines appear to offer less protection, although CDC officials said booster shots strengthen that protection.

Dr. Anthony Fauci, the nation’s top infectious disease expert, said Wednesday there is no need, for now, for an omicron-specific booster shot. The two-dose mRNA vaccines, the Pfizer and Moderna shots, still appear to offer considerable protection against hospitalization from omicron, Fauci said.

“If we didn’t have these tools, I would be telling you to really, really be worried,” Fauci said.

Jeff Zients, the White House coronavirus response coordinator, said the U.S. has the tools to fight the virus, including omicron, and “there is no need to lock down.” With vaccines available now for 95% of Americans, “we know how to keep our kids in schools and our businesses open. And we’re not going to shut down.”

Health officials called on Americans to get vaccinated, get their booster shots, wear masks indoors and get tested before traveling and before holiday gatherings.

“Hospital capacity is already at a breaking point in many states because of severe cases of COVID-19,” Michael Fraser, CEO of the Association of State and Territorial Health Officials, said in a statement.

Given the high level of transmission, MacInnis said there will undoubtedly be severe cases.

“No matter how severely it affects healthy, fully vaccinated and boosted populations, it will hit the most vulnerable among us the hardest still,” she said. “So the elderly, the immunocompromised, other vulnerable populations will still be at greatest risk and still bear the brunt of this.”