Exactly two weeks after the Thanksgiving holiday and its associated gatherings of family and friends, Los Angeles County is seeing an increase in COVID-19 cases, the public health director said this week, calling the trend a possible start of yet another winter surge of infections.

In an online briefing, Barbara Ferrer said Thursday 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.

Long Beach has also seen its numbers increase, rising to 11.5 cases per 100,000 residents from 9.1 cases on Dec. 2.

The county’s 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.