Dr. Anthony Fauci, director of the NIH National Institute of Allergy and Infectious Diseases, testifies about Coronavirus, COVID-19, during a Senate Committee on Health, Education, Labor, and Pensions hearing on Capitol Hill in Washington, DC, March 3, 2020. Saul Loeb | AFP | Getty Images

A top U.S. health official said the worst is yet to come with the coronavirus outbreak that has already infected more than 1,000 people across the nation and resulted in at least 31 deaths. "I can say we will see more cases and things will get worse than they are right now," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told the House Oversight and Reform Committee on Wednesday at a hearing on the nation's preparedness for the outbreak. Fauci said COVID-19 is at least 10 times "more lethal" than the seasonal flu, even if the mortality rate drops far below the World Health Organization's current estimate of 3.4%.

The 2003 SARS epidemic, which began in November 2002 and ran through July 2003, had a mortality rate of 9% to 10%, he said, meaning roughly 10% of the people who caught it eventually died. Since COVID-19 emerged in China two and a half months ago, "it clearly is not as lethal ... but it certainly spreads better," he said, adding seasonal flu has a mortality rate of 0.1%. The WHO's estimated mortality rate for COVID-19 started off at 2%, Fauci said. If you count all the estimated cases of people who may have it but haven't been diagnosed yet, the mortality rate is probably closer to 1%, he said, "which means it's 10 times more lethal than the seasonal flu." The H1N1 "swine flu" epidemic in 2009 was even less lethal than regular seasonal flu, Fauci said. While COVID-19 cases are slowing in China, where the outbreak originated in December, it's picking up pace across other parts of the world. Italy has the most cases outside of China with roughly 10,150 infections, followed by Iran with 9,000 infections and South Korea with more than 7,700, according to data compiled by Johns Hopkins University. In the U.S., the number of cases erupted over the last week to more than 1,030 spread across at least 36 states, according to Johns Hopkins. The COVID-19 outbreak is just weeks underway in the U.S. How much worse it gets depends on two things, Fauci said: containing the influx of infected people coming from other countries and containing local outbreaks within the U.S. "Bottom line is it's going to get worse," he added.

Once there's widespread community transmission, or if health officials can't trace back the origin of localized outbreaks, that's when the country won't be able to "effectively and efficiently contain it," Fauci said. Local and state officials have taken various measures to try to contain the virus, including closing schools, canceling parades and other public events, and even, in New York, mobilizing the National Guard to the "containment zone" in the hard-hit community of New Rochelle. When pressed by lawmakers for an estimate of eventual fatalities in the U.S., Fauci said it will be "totally dependent upon how we respond to it." "I can't give you a number," he said. "I can't give you a realistic number until we put into the factor of how we respond. If we're complacent and don't do really aggressive containment and mitigation, the number could go way up and be involved in many, many millions." Dr. Robert Redfield, the director of the Centers for Disease Control and Prevention, confirmed that COVID-19 has now killed 31 people in the U.S. Globally, there are more than 119,000 confirmed cases and at least 4,291 deaths, according to Johns Hopkins. President Donald Trump summoned the panel of witnesses to an emergency meeting at the White House later Wednesday, cutting the hearing short, Committee Chairwoman Rep. Carolyn Maloney said at the top of the hearing. "This morning we were informed that President Trump and Vice President [Mike] Pence have called our witnesses to an emergency meeting at the White House. We don't know the details, just that it's extremely urgent," said Maloney, D-N.Y. The White House didn't immediately respond to CNBC's request for comment. White House officials later told reporters the meeting was scheduled yesterday as "part of the administration's ongoing whole-of-government response to the coronavirus."