Vaccine hesitancy in the U.S. has proven to be a stubborn issue that is holding back America's ability to defend itself against the particularly infectious Delta variant in certain parts of the country.

In addition to unvaccinated adults themselves, two demographics are particularly vulnerable as the number of confirmed COVID-19 cases rises.

“Children under 12 don’t have a vaccine yet,” Dr. Heather Yeo, associate professor of surgery and health care policy and research at Weill Cornell Medical College, said on Yahoo Finance Live (video above). “And there are individuals who are immunocompromised who can’t always take vaccines or don’t have an active response to vaccines. We’re worried that they are going to be patients put at risk if other people who have the opportunity aren’t vaccinated.”

About 67.4% of Americans over the age of 12 are fully vaccinated and 66.4% have received at least one dose, according to the latest CDC data. President Biden has stated that he is hoping to have a vaccine approved for children under 12 in the coming months.

"It doesn't have anything to do with size," CDC Adviser Dr. William Schaffner told CNN when asked about the different in vaccines for children and adults. "It has everything to do with maturity of the immune system, and that doesn't correlate one-to-one with the size of the child."

So far, only Pfizer (PFE)'s vaccine has been approved for children between the ages of 12-18.

“One of the biggest fears is that children are vectors even though many of them aren’t getting very sick,” Yeo said. “They do have the potential to transmit, particularly in areas where the vaccination rates are low. I’m lucky. I’m in New York and I think 75% of our population is vaccinated. So we’re starting to get closer to potential herd immunity for those individuals that aren’t vaccinated.”

'They need a host, and we're the host'

Younger people have proven to be less vulnerable to transmitting coronavirus as well as hospitalization and death once infected, though each new variant creates the possibility that the effects on children could change.

Children accounted for 16.8% of new confirmed COVID-19 cases for the week ending July 22, according to the American Academy of Pediatrics. (Overall, children represent about 14.2% of all confirmed cases.)

Vaccines have thus far been very effective against the worst outcomes of the COVID-19, including during the recent surge of Delta variant cases. And the ongoing transmission of coronavirus, particularly among unvaccinated individuals, gives the virus more opportunities to continue mutating.