"Don't host large parties and don't attend a party if you're invited," Dr. Barbara Ferrer said. "It isn't worth the risk you run, and it certainly isn't worth the risk you're creating for our collective recovery journey."
Ferrer said such gatherings are a breeding ground for the virus among younger residents who can in turn infect older and more vulnerable residents.
"Gatherings are simply not allowed at this point under the health officer order,'' Ferrer said. "Because they create a lot of risk for transmission at activities that really are not essential. These parties and gatherings with people not in your household hurt all of us as we try to reduce our case rates so we can get our children back to school and get other adults back to their jobs."
RELATED: Hundreds of maskless revelers seen at Holmby Hills mansion
She said she believes the most important goals for the county at the moment is to get students back to school and for people to return to workplaces, which would require putting parties on hold.
Ferrer expressed frustration at people who would host or attend a party.
"I do think an equally important question to ask is why so many people are willing to put our entire community at risk during this unprecedented pandemic,'' she said.
1 dead, 4 injured after shooting near massive mansion party in Beverly Crest where many seen without masks
Her announcement during the county's coronavirus briefing comes after reports of a large party at a mansion in Beverly Crest, which later ended in a fatal shooting.
Hours after her the briefing, AIR7 HD was over a large party in Holmby Hills where many guests were not wearing masks or practicing physical distancing.
L.A. Mayor Eric Garcetti, in an attempt to crack down on parties, announced the Department of Water and Power will be authorized to shut off utility service at venues that host large gathering. The shutdown process can take up to 48 hours and would apply to property that repeatedly violated orders.
Meanwhile, L.A. County health officials say the recent downturn in COVID-19 hospitalizations continues, but they point out that numbers among the county's younger population are rising.
Nearly 60% of COVID-19 cases in LA County are among residents in 18-49 age group
Over half of new cases in the county are in residents between the ages of 18 and 49, officials announced Wednesday, with those 30-49 driving up most of the cases.
Since June, case rates for the 30-49 age group have nearly tripled to a high of 1,122 cases per 100,000 people, according to the L.A. County Department of Public Health.
Younger people across the county are also being hospitalized with the virus more than before -- individuals in the 30-49 age group account for 25% of hospitalized patients.
The hospitalization rates for patients 18-29 has more than doubled since April, actually matching the rate of people over the age of 80. By comparison, hospitalizations for that elderly group has fallen in half since its peak in April.
Although people in those younger age groups might not see severe effects of the virus, those figures are still concerning to county public health officials.
"It's also important to remember that although you, as an individual, particularly a younger adult, may not suffer these devastating consequences from COVID-19, you could infect someone you love, and that could be a relative or a friend, and you could infect someone in your community, who could get very sick and unfortunately pass away," Ferrer said.
Officials have also noted an outbreak at three USC fraternities that has sickened 45 people, and another one at UCLA where at least eight football players have tested positive.
City News Service contributed to this report.