Anthony Fauci says the cause of more than 80% of new Covid-19 cases in the U.S. is the Delta variant.

The country's top infectious disease expert urged citizens to get vaccinated to help prevent hospitalization, serious side effects and death.

Fauci said authorized vaccines used in the U.S. are more than 90% effective in preventing serious infections. During the hearing with the U.S. Senate, the head of the nation's coronavirus response efforts faced intense scrutiny from politicians.

Republican Senator Rand Paul accused Fauci of lying about alleged funding provided by the National Institute of Health to China's Wuhan Institute of Virology. The senator said such a transaction would be a violation of U.S. criminal codes, and those found guilty could face up to five years in prison. He said the alleged funding might have been used by China to develop the coronavirus.

Fauci outright denied the accusations and said he was not aware of such a transaction.

"Senator Paul, I have never lied before the Congress...(and) you do not know what you're talking about," Fauci said. 

The issue of whether or not the virus originated from a lab leak in Wuhan continues to be a point of contention between U.S. politicians. Republicans have repeatedly called on further inquiries into the matter.

In May, U.S. President Biden called on a more comprehensive investigation into the origins of the virus. Virology experts have mostly theorized that the virus may have originated in animals - possible bats - before passing to humans. However, some have not completely ruled out the lab leak theory.

The U.S. has recently seen a surge in new infections fueled by the Delta variant of the coronavirus. Fauci said the variant has now become the dominant version of the virus in the U.S and many countries. The variant was first detected in India earlier in the year. The variant is now present in more than 90 countries worldwide.