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.

The American Medical Association, the nation’s largest association of physicians, has criticized the Centers for Disease Control and Prevention's new quarantine and isolation guidance for COVID-19, saying the recommendations "are risking further spread of the virus."

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.

"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 American Medical Association's president, Dr. Gerald E. Harmon, said in a statement Wednesday night.

Harmon referenced data cited by the CDC in its rationale for shortening the isolation period, which estimates 31% of people remain infectious five days after a positive COVID-19 test, suggesting that data proves thousands of Americans could return to their lives while still infected.

"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," Harmon said. "Physicians are concerned that these recommendations put our patients at risk and could further overwhelm our health care system."

Harmon said a negative COVID-19 test should be required for ending isolation after a positive test, as reentering society without knowing whether an individual is still positive ultimately risks further transmission of the virus.

Although test availability remains an issue nationwide, Harmon also called on the Biden administration to ramp up production and distribution of tests, adding that "a dearth of tests at the moment does not justify omitting a testing requirement to exit a now shortened isolation."

-ABC News' Arielle Mitropoulos