Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said on Tuesday many experts believe more people in the United States have died from COVID-19 than those who have been reported.

"Most of us feel that the number of deaths is likely higher than that number," Fauci told a Senate hearing.

"Given the situation, particularly in New York City, when they were really strapped with a very serious challenge to their health care system, that there may have been people who died at home... who are not counted as it because they never really got to the hospital," he said.

Fauci added he was not sure "exactly what percent higher" the real death toll could be.

Fauci also warned that if states disregard COVID-19 guidelines for safe reopening, those actions could "turn the clock back" on stemming the tide of coronavirus infections.

There could be "really serious" consequences if states and areas reopen prematurely, he said.

While the coronavirus will not simply disappear this fall, Fauci said the threat of a possible second wave can be mitigated by aggressive testing efforts and health care preparedness. He said that the second wave is "entirely conceivable and possible."

If there is not an "adequate" response by the United States in the fall, more infections and deaths are on the horizon, he said.

"We run the risk of having a resurgence. I would hope by that point in time in the fall that we have more than enough to respond adequately, but if we don't, there will be problem," Fauci said.

When asked whether there will be treatments or even a vaccine available to help reopen universities in the fall term, Fauci said students might feel safest if there was a vaccine for the coronavirus, but it is a "bridge too far" to think a vaccine or treatment will be ready by the time classes start this fall.

"If this were a situation where we had a vaccine, that would really be the end of that issue in a positive way," Fauci said. "We don't see a vaccine playing in the ability of individuals to get back to school this term."