WASHINGTON, United States -- The United States is on a similar trajectory to a Delta variant outbreak like the one seen earlier this year in the United Kingdom, said Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, on Tuesday.

"Since an acceleration of vaccines doesn't give a result until several weeks after, we are already on a trajectory that looks strikingly similar to the sharp incline that the UK saw," Fauci said during a discussion hosted by the Center for Strategic and International Studies.

"In order to make sure that by the time we get into the fall we don't continue to accelerate but turn around and start coming down acutely, we've got to get those 93 million people who are eligible to be vaccinated, who are not getting vaccinated," he said.

COVID-19 variants are likely to have evolved in the bodies of people who are immunosuppressed, Fauci said.

"Variants, we all know, have emerged because of the pressure that the human immune system has put on the virus, very likely from people who are immunosuppressed wound up getting infected, and had virus in them for days and days and days before they cleared it and/or died, and then essentially led to the emergence of a variant," Fauci said.

As of Monday, 70 percent of U.S. adults aged 18 and older had received at least one vaccine dose while 60.6 percent of American adults were fully vaccinated, according to the U.S. Centers for Disease Control and Prevention (CDC).

The current seven-day average of daily new cases increased 64.1 percent compared with the previous seven-day data, according to a CDC weekly report. Nationally, the proportion of cases attributed to B.1.617.2 (Delta) is predicted to increase to 82.2 percent.