COVID-19 updates: Classes in Chicago canceled for 4th day

COVID-19 vaccines dramatically reduce the risk of severe illness and death from the virus, according to a new study from the Centers for Disease Control and Prevention published Thursday.

The study looked at 1.2 million fully vaccinated adults who received either two shots of the Pfizer-BioNTech or Moderna vaccines or one shot of the Johnson & Johnson vaccine.

Just 0.18% of patients had breakthrough COVID-19 infections, and 0.015% developed serious illness that led to hospitalization, ICU admission, mechanical ventilation, intubation or death.

The small portion of people who did become seriously sick or die of COVID-19 after being vaccinated were primarily older adults, immunocompromised people or those living with multiple underlying medical conditions. 

The study was conducted before the emergence of the omicron variant, which appears to more easily evade --- at least partially -- the protection offered by vaccines.

-ABC News' Sony Salzman

An estimated 126,000 Americans are currently hospitalized with COVID-19, according to new data from the U.S. Department of Health and Human Services.

Of those patients, about one-sixth -- or 21,000 -- are in intensive care units.

The spread of the omicron variant is pushing the U.S. closer to the hospitalization record set last winter when 140,000 patients with the virus were hospitalized .

Meanwhile, the country is averaging 554,000 new COVID-19 cases every day, a five-fold increase from a month ago, federal data shows.

Over the last week, the U.S. has reported about 3.88 million COVID-19 infections, which averages out to about six Americans testing positive every second.

-ABC News' Arielle Mitropoulos

More than 800 staff members at fire and police departments in Los Angeles have tested positive for COVID-19, Mayor Eric Garcetti announced Thursday.

Currently, 505 police officers and 299 firefighters are quarantining after receiving positive results.

"These are big numbers. Numbers that are reflecting the staffing challenges that all of us face," Garcetti said during a press conference outside a fire station.

He added that five out of six employees in both departments are fully vaccinated, complying with the vaccine mandate set in place for city employees.

Garcetti said that despite the number of officers and firefighters absent from work, the city is still protected.

“Both LAPD and LAFD have maintained staffing levels that are needed to keep Angelenos safe, and we have maintained staffing levels to make sure you, your family, our communities are safe," he said.

Global cases of COVID-19 "increased sharply," the World Health Organization wrote in its weekly epidemiological update published Thursday.

Cases of the virus increased 71% during the week ending Jan. 2 compared to the previous week, meaning there were 9.5 million new infections of COVID-19 reported around the world, according to the WHO.

The Americas saw the largest increase at 100% followed by Southeast Asia at 78% and Europe at 63%.

The United States saw the most COVID-19 cases last week followed by the United Kingdom, France, Spain and Italy.

The WHO added that there was a 10% decrease in the number of new deaths compared to the week prior with more than 41,000 virus-related fatalities recorded.