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.

An estimated 1.3 million people in the United Kingdom are suffering from long COVID, according to new data published Thursday by the UK's Office for National Statistics.

Long COVID is a condition in which patients who recover from the virus still experience symptoms for weeks -- or even months or years -- later.

The ONS said the figure, which is based on self-reported symptoms, equates to 2% of the population living with long COVID.

Fatigue was the most common symptom, reported by 51% of those surveyed. Other common symptoms included loss of smell (37%), shortness of breath (36%) and difficulty concentrating (28%).

About 64% said their long COVID symptoms made it difficult for them to carry out day-to-day activities.

Long COVID was most commonly reported in those between ages 35 and 69, females, people living in poorer areas or those working in health care or education, according to the ONS.

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

Data released by the Centers for Disease Control and Prevention from the weekend shows that the U.S. is now averaging nearly half a million new cases a day.

About a year ago, the U.S. was reporting an average of about 216,000 new cases a day. However, compared to last winter, hospital admissions and daily deaths are notably lower.

Over the last week, the U.S. reported more than 3.4 million new cases -- roughly averaging out to nearly six Americans testing positive every second.

-ABC News' Arielle Mitropoulos