Dr. Anthony Fauci, the nation’s top infectious disease expert, said a growing body of evidence suggested that the omicron variant of the coronavirus was causing less serious illness than its predecessors, but he warned against complacency, saying the variant’s lightning-speed spread across the United States would likely lead to a perilous spike in hospitalizations among the unvaccinated and could overwhelm the country’s health systems.

Speaking on ABC’s “This Week” today, Fauci said recent data out of Scotland, England and South Africa has been filling in the fragmentary portrait of omicron, which has spread across much of the world and overtaken the delta variant in the United States in the month since it was first identified by scientists in South Africa.

“Even though we’re pleased by the evidence from multiple countries — it looks like there is a lesser degree of severity — we’ve got to be careful that we don’t get complacent about that,” Fauci said, noting that there were still tens of millions of unvaccinated Americans. “Those are the most vulnerable ones when you have a virus that is extraordinarily effective in getting to people and infecting them the way omicron is.”

Last week, scientists at the University of Edinburgh in Scotland reported that people infected with omicron were almost 60% less likely to be hospitalized than those infected with delta. Another study from Imperial College London found that individuals infected by omicron were 15% to 20% less likely to go to an emergency room with severe symptoms and 40% less likely to be hospitalized.

Despite such encouraging data, Fauci said the nation’s low vaccination rate — only 62% of Americans are fully vaccinated — would likely dilute the benefits of omicron’s reduced virulence. “When you have such a high volume of new infections, it might override a real diminution in severity,” he said.

Nearly 71,000 Americans are currently hospitalized with COVID-19, up 10% from the previous week but still well below previous peaks.

That said, the nation’s medical infrastructure is dangerously frayed two years into the pandemic as hospitals contend with staff shortages fueled by burnout and early retirements. Experts also worry about a coming wave of omicron infections that could sideline an untold number of nurses and doctors.

Despite an alarming spike of cases in the United States — the seven-day average of new daily cases has surpassed 197,000, a 65% jump over the last 14 days — government data show that vaccination is still a strong protector against severe illness. Unvaccinated people are five times more likely to test positive and 14 times more likely to die of COVID than vaccinated patients, according to the Centers for Disease Control and Prevention.

This article originally appeared in The New York Times.

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