As COVID-19 cases creep up again across the country, federal officials and epidemiologists say they're worried we could hit another tipping point, leading to a fourth significant surge of infections, hospitalizations and deaths.

"We're skating on a knife's edge right now," said Nicholas Reich, a biostatistician at the University of Massachusetts Amherst.

Average daily reported cases are up 10% compared to a week earlier, data from the Centers for Disease Control and Prevention shows, with more than 30 million COVID-19 cases reported since early last year. Hospitalizations and deaths, which usually lag cases by a few weeks, have inched upward as well, after a decline and plateau that began in early January.

Reich and others say they expect that the immunity from natural infections plus the successful rollout of vaccines, which are now reaching nearly 3 million people a day, will help moderate this surge.

But Dr. Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said in a White House briefing with media Monday that she's anxious about what the next few weeks could bring.

"Right now I’m scared," she said in what she described as an off-script moment of candor.

The vaccine rollout is giving people hope and the spring weather is making everyone even more restless, she acknowledged, but it's too soon for Americans to let down their guard, return to travel and stop using the precautions – such as mask-wearing, social distancing and hand-washing – that are known to be protective.