The seven-day positivity rate at the U.S. Capitol COVID-19 testing center has increased from 1% to 13%, according to a letter sent from the Office of the Attending Physician to congressional offices on Monday.

Most cases have been breakthroughs among people who are fully vaccinated and have not “led to hospitalizations, serious complications, or deaths, attesting to the value of coronavirus vaccinations," the letter reads.

The letter also states that about 61% of COVID-19 infections at the Capitol have been linked to the omicron variant, and 38% are linked to the delta variant.

The OAP advised congressional offices to telework as much as possible and that "blue surgical masks, cloth face masks and gaiter masks must be replaced by the more protective KN95 or N95 masks."

New York City Mayor Eric Adams insisted that schools will stay open for in-person leaning despite the rapidly rising number of COVID cases.

“We’re really excited about the opening of our schools,” he said during a speech outside Concourse Village Elementary School in the Bronx on Monday. “We want to be extremely clear: the safest place for our children is a school building.”

As part of a plan to curb rates of infection, 1.5 million rapid at-home test kits are being being distributed to schools.

Students and adults who exhibit symptoms or have been exposed to a positive case will be required to have two negative tests over the course of five days before they can return to classes.

On Sunday, Adams told ABC's "This Week" that parents should "fear not sending [kids] back" to school after the winter break.

Meanwhile, several other school districts across the country, including Atlanta, Cleveland, Newark and Milwaukee, have opted for virtual learning for at least the first week of the new semester.

More than 100,000 Americans are currently hospitalized with COVID-19, according to data from the U.S. Department of Health and Human Services.

A total of 103,329 people are receiving medical care, which is an increase of 130% from the number recorded two months ago.

Of those patients, more than 17% -- about 18,000 -- are in intensive care units.

The newly updated figure is just shy of the hospitalization peak seen during the summer wave fueled by the delta variant, when 104,000 Americans were hospitalized with COVID in early September.

-ABC News' Arielle Mitropoulos

The U.S. Food and Drug Administration authorized booster shots of Pfizer-BioNTech's COVID-19 vaccine in children between ages 12 and 15 on Monday.

Booster shots have been touted as a key tool in fighting the surge in COVID cases linked to the omicron variant, which has shown an ability to -- at least partially -- evade protection offered by two doses.

The FDA also shortened the wait period for adults and adolescents to receive boosters from six months down to five months.

In addition, the agency authorized COVID booster shots for children aged five to 11 who are immunocompromised.

Dr. Rochelle Walensky, director of the U.S. Centers for Disease Control and Prevention, is standing by her agency's new quarantine and isolation guidance for COVID-19, which the nation’s largest association of physicians has criticized as "confusing."

The CDC updated its guidelines on Dec. 27, saying asymptomatic people who test positive for COVID-19 should self-isolate for five days rather than 10. In a scathing statement released Wednesday night, the American Medical Association (AMA) said the new recommendations "are risking further spread of the virus."

"The American people should be able to count on the Centers for Disease Control and Prevention (CDC) for timely, accurate, clear guidance to protect themselves, their loved ones, and their communities. Instead, the new recommendations on quarantine and isolation are not only confusing, but are risking further spread of the virus," the AMA's president, Dr. Gerald E. Harmon, said in the statement. "With hundreds of thousands of new cases daily and more than a million positive reported cases on January 3, tens of thousands -- potentially hundreds of thousands of people -- could return to work and school infectious if they follow the CDC’s new guidance on ending isolation after five days without a negative test."

But Walensky defended the new guidance, telling ABC News' Cecelia Vega in an interview Friday on "Good Morning America" that the CDC "make[s] these recommendations in the context of science, in the context of ongoing epidemiology and in the context of what is feasible in collaboration with our public health and local and state public health partners."

"I've read the AMA statement and I have deep respect for the AMA," Walenskey said, "but I will also say we've heard a lot of support for ongoing guidance from public health partners, from other clinical and laboratory partners as well."

Walensky referenced "dozens of studies" that have shown a COVID-19 patient is most infectious in the one to two days before symptoms and the two to three days after symptoms.

"So by five days after your symptoms, the vast majority of your contagiousness is really behind you," she said. "And what we say at day five then is, are your symptoms gone? Are you feeling better? Is your cough gone, sore throat gone? And if so, then it is safe to go out if you are wearing a mask all the time. And that means not going out to restaurants, not going out to gyms, not going out and visiting grandma, but really conscientiously wearing your mask for those last five days."

Walensky noted that people may choose to take an antigen or at-home test at day five, if they have access to one. But it's not required.

"If that test is positive, you should stay home for those extra five days. But if that test is negative you should go out and continue to wear your mask," she said. "You know, these tests are actually not authorized for the purpose of evaluating contagiousness."