Both Republicans and Democrats clash with RFK Jr. on vaccines, CDC turmoil

During the hearing, Sen. Roger Marshall, a Republican from Kansas, pointed to a sign that said "76 JABS" noting that is how many vaccines children in the U.S. receive by age 18.

Children and teens may receive multiple vaccines before adulthood; these are often broken up into multiple doses. These multiple doses do not overwhelm a baby's immune system, the CDC says.

Vaccines are given early in life because that is the time when people are especially susceptible to diseases.

Some babies may also receive vaccines as a combined shot that reduces the number of needles they get at one time.

-ABC News' Youri Benadjaoud and Dr. Jade Cobern

The hearing has ended after nearly three hours. At many times contentious, Kennedy was grilled on everything from vaccine availability to his staffing shakeups at the CDC. Several Republicans on the panel expressed concern on vaccines, while Democrats accused Kennedy of breaking pledges he made during his confirmation hearing back in January.

Kennedy is currently not testifying under oath after Sen. Crapo denied a request to do so.

At the beginning of the hearing, Sen. Wyden asked that Kennedy be formally sworn in as a witness.

"I will personally object and will reject your request," Crapo said. "We will treat this witness as we treat all of the other administration witnesses who come before us."

Minnesota Democratic Sen. Tina Smith slammed Kennedy over his comments following last week's deadly school shooting.

During a Fox News interview following the shooting, Kennedy said he was interested in looking at "the potential contribution of some of the SSRI drugs and some of the other psychiatric drugs that might be contributing to violence."

The investigation into the shooting is still ongoing and there is no evidence that SSRI drugs were a factor in the shooting.

"You have no knowledge of that because you have no evidence of a connection," Smith said.

Kennedy claimed that she was making it up.