Washington DC – The White House will fall short of its goal of getting at least 70% of the adult population partially vaccinated by July 4, White House officials announced Tuesday, largely because younger adults are refusing the Covid-19 shot.

The administration has hit its 70% partially vaccinated target for Americans ages 30 and older and is expected to reach that threshold for Americans ages 27 and older by July 4, said Jeff Zients, the head of the White House Covid-19 response team. But it will take a few more weeks to get Americans ages 18 to 26 up to that level.

"The reality is, many younger Americans have felt like Covid-19 is not something that impacts them and have been less eager to get the shot," Zients told reporters on Tuesday. NBC News first reported the delay.

The White House still is planning a celebration for the holiday weekend featuring health care workers and members of the military. Zients said President Joe Biden will host about 1,000 people on the White House lawn.

Members of the administration have been crisscrossing the country to promote the vaccination campaign. On Tuesday, first lady Jill Biden is scheduled to join Rep. Bennie Thompson, a Democrat from Mississippi, for a tour of a vaccination site in Jackson, Mississippi. Then the first lady will head north to Nashville, Tennessee, for a similar event with country music star Brad Paisley at the Ole Smokey Distillery.

Vaccine clinics at places like breweries and distilleries, as well as incentives for getting the vaccines, have had some success, but the White House plans to double down on its efforts to reach younger populations.

"We are not stopping at 70%, and we are not stopping on July 4," Zients said.

National Institute of Allergy and Infectious Diseases Director Anthony Fauci said the ultimate goal is "crushing the outbreak completely in the United States." This will require an intense focus on convincing unvaccinated younger people to get the shot.

Fauci said highly contagious virus variants could spread rapidly among unvaccinated individuals. But Fauci also predicted that any resurgence this fall will be limited to regional pockets where vaccination rates are low.

"I don’t foresee what we refer to as a surge as we have known it," he said.

The country is now averaging 10,350 new Covid cases per day over the past week, a decrease of nearly 18% over the prior seven-week average. Centers for Disease Control and Prevention Director Rochelle Walensky said that deaths also continue to decrease, averaging 270 per day.

Young adults in Generation Z have been among the most vaccine-hesitant groups in America, and unlike people in other age groups, their reluctance to get the shot has increased over time.

The young and healthy were not prioritized for vaccinations in the early days of the Covid-19 pandemic, and many young adults said they didn’t see a need to rush as they watched case numbers fall, mask restrictions let up, and life begin to return to normal.

The Biden administration recently began to take notice of the younger generation’s reluctance, and Biden encouraged young adults to get the shot.

"For young people who may think this doesn’t affect you, listen up, please: This virus, even a mild case, can be with you for months. It will impact on your social life," Biden said this month. "It could have long-term implications for your health."

Public health experts caution that even though younger adults have a lower risk of contracting serious disease from Covid-19, remaining unvaccinated could have serious consequences for those who have lingering effects, known as "long Covid."

The unvaccinated also further the spread of the virus and fuel new variants.

"If we get vaccinated now, it's hard for the new strains to get in here, but if they're in here and ... really are easy to transmit, then there's more of a risk," said Arthur Caplan, a professor of bioethics at New York University Langone Medical Center.

The highly contagious Delta variant, which first originated in India, has infected many young people, but Amesh Adalja, a physician and a Johns Hopkins Center for Health Security senior scholar, says this is because more young people are unvaccinated, not because the variant specifically targets young people’s immune systems.

Fauci noted that in the United Kingdom, young, unvaccinated people are driving the spread of the B.1.1.7 variant, also known as the Alpha variant. The same thing is happening in the United States with the Delta variant. But studies prove that two doses of the Pfizer-BioNTech vaccine are extremely effective at preventing hospitalization and severe disease from these variants.

Adalja also argued that vaccinating younger populations isn’t as important as vaccinating older, more at-risk people, and the Biden administration has already achieved its original goal of flattening the curve. Over 77% of people over age 65 are fully vaccinated, according to the Centers for Disease Control and Prevention.

"If there was a pocket of 75 and up that didn't want to get vaccinated, that’s much more of a problem than it being in younger individuals who are not likely to require hospitalization," Adalja said.

Black and Hispanic communities have also fallen behind when it comes to Covid-19 vaccinations, according to recent Kaiser Family Foundation data. Vaccination rates are particularly lagging in Southern states with high rates of vaccine hesitancy.