LOS ANGELES (CNS) — Exactly two weeks after the Thanksgiving holiday and its associated gatherings of family and friends, Los Angeles County is seeing a resulting increase in COVID-19 cases, the public health director said Thursday, calling the trend a possible start of yet another winter surge of infections.

In an online briefing, Barbara Ferrer said the increase was visible by Dec. 1, when the county's seven-day average daily number of new cases topped 1,000 — a 19% increase from the previous week. She also noted a resulting increase in hospitalizations, with the daily number of COVID patients nearing roughly 600.

"We do expect increases to continue on the heels of our Thanksgiving gatherings, but already, based on trends, we are looking at possible beginnings of a winter surge," Ferrer said.

She said the county's current average daily rate of new infections has risen to 13 per 100,000 residents, up from 8 per 100,000 residents a week ago. The seven-day cumulative rate of infections rose to 113 per 100,000, moving the county back into the category of "high" transmission as defined by the U.S. Centers for Disease Control and Prevention. The county was previously in the less-severe "substantial" transmission category. That category requires a county to have a cumulative seven-day transmission rate of less than 100 cases per 100,000 residents.

Ferrer said the county's case increase was also reflected in schools.

"In the week following the Thanksgiving break, cases among students in particular rose to their highest level since late September," Ferrer said. "If, as we suspect, this increase in cases reflects transmission that took place during holiday gatherings, we should consider this an early warning about the upcoming December holiday."

Ferrer said infections among students are likely due to Thanksgiving gatherings, because transmission at schools remains low thanks to strict infection-control measures on campus, such as regular testing and mandatory mask-wearing.

She acknowledged that with the widespread availability of vaccines and the benefit of more experience preventing and treating infections, the county can be considered to be "much better off" than last winter. But she insisted, "all increases in cases are worrisome."

"I don't want to downplay the fact that we continue to now be back in what the CDC classifies as the tier of 'high' transmission," she said. "So we have a lot of community transmission going on. And when you have a lot of community transmission going on and there's lots and lots of opportunities of people intermingling, you run the risk of these numbers just continuing to grow. And every time they grow and we see more and more cases, we all know it results unfortunately in a higher number of people that will end up in the hospital and tragically pass away."

COVID vaccines will likely limit the impact of a major winter surge on hospitals and the county's overall health-care system, Ferrer said, noting that while vaccinated people may get infected, they are less likely to become severely ill and require hospitalization. But she said more people need to get the shots to prevent strain on hospitals.

"There's a lot we all need to do to slow down transmission and that we're obviously not all doing," she said, urging vaccinated residents to get booster shots to counter waning immunity from the original shots. She said 5 million people in the county are eligible for booster shots, but only 1.6 million booster doses have been administered.

"All of you who are waiting, please don't wait any longer," she said. "The boosters are essential to add additional protection."

The county reported another 15 COVID-19 deaths on Thursday, raising the death toll to 27,288.

Another 1,718 new infections were also reported, giving the county a pandemic total of 1,541,886.

According to state figures, there were 667 COVID-19-positive patients in Los Angeles County hospitals as of Thursday, the same as Wednesday. The number of those patients being treated in intensive care was 158, up from 151 a day earlier.

The rolling average daily rate of people testing positive for the virus was 1.4% as of Thursday.

According to the most recent figures, 83% of county residents aged 12 and over have received at least one dose of vaccine, and 75% are fully vaccinated. Of all eligible residents aged 5 and over, 77% have received at least one dose, and 69% are fully vaccinated.

Of the more than 6.15 million fully vaccinated people in the county, 84,931 have tested positive, or about 1.38%. A total of 2,798 vaccinated people have been hospitalized, for a rate of 0.046%, and 537 have died, for a rate of 0.009%.

While the county Department of Public Health has identified a total of four cases of the new omicron variant of COVID-19 — and Long Beach has confirmed one additional case — Ferrer said the delta variant remains the dominant strain of the virus in the county, accounting for more than 99% of cases that undergo genetic sequencing.

Ferrer said the county is now conducting sequencing of 25% of all positive cases to identify COVID variants.