Communities such as Lancaster, Palmdale, Studio City and Santa Clarita are among the 10 places in the county that are seeing increased rates of new infections during a two-week period that ended Nov. 6.
According to the county health department, seven of the communities on that list had vaccination rates that exceeded the countywide rate, but officials insist that does not mean the vaccines are not working.
"If you're not vaccinated, you've got a much higher risk of ending up infected, ending up in the hospital and tragically passing away. That's crystal clear and it hasn't really changed for months now," Health Director Barbara Ferrer said during a media briefing.
Officials are studying the data to determine what is driving higher case rates in those communities.
The top two communities on the list with the highest new case rates -- Lancaster and Palmdale -- have below-average rates of fully vaccinated residents, at 58% and 66%, respectively. But Studio City, with the third-highest new case rate, has a 79% vaccination rate, and Santa Clarita, placing fourth on the list, has a 75% vaccine rate.
The countywide number of fully vaccinated residents is 73%.
Of the other communities on the top 10 list of highest new-case rates, only Willowbrook, at 62%, falls below the countywide vaccination rate.
"Some of our communities that have right now these higher case rates are in fact communities that have really decent coverage in terms of vaccination ... and they still have a problem with high case rates," Ferrer said.
Lancaster, Palmdale, Studio City, Santa Clarita, the wholesale district, downtown L.A., Stevenson Ranch, Venice, West Hills and Willowbrook were the communities on the list.
She said a variety of factors could be at play in different communities, among them the possibility that some areas had large numbers of people who were never previously infected with COVID-19 and remain unvaccinated, leading to higher current infection numbers.
"That certainly is possible, although we have to look at more data to draw that conclusion," Ferrer said.
But she noted that the median age of people becoming infected ranges from 26 to 36, meaning young people are driving the numbers.
"I will say the one thing that does jump out -- the average ages were very low in all these communities," she said. "This is, essentially, in the communities with the highest rates, this is a pandemic that is in fact fueled by younger people."
City News Service contributed to this report.