(CNN) A teen girl in Southern California has died of the coronavirus, Orange County health officials announced Wednesday.
The girl had "significant underlying medical conditions," officials said in a news release without providing further details about the child or her health conditions.
"We are deeply saddened by this loss of life and send our condolences to her friends and family during this very difficult time," Dr. Clayton Chau, the acting county health officer, said in a statement.
Across the state, more than 638,000 people have tested positive for the virus, including 63,000 cases among children younger than 18, state data show.
The number of Covid-19 cases among children nationwide recently increased 90% over four weeks, according to data released last week by the American Academy of Pediatrics. "We've had 90 deaths in children in the United States already, in just a few months," Dr. Sean O'Leary, vice-chair of the AAP Committee on Infectious Diseases, told CNN last week.
Orange County, with more than 43,000 cases, has the third highest number of reported infections in California, following Los Angeles and Riverside Counties. Orange County health officials have reported 833 coronavirus-related deaths.
Gov. Gavin Newsom placed Orange County on the state's watch list in late June and said it was seeing a rise in hospitalizations driven by public gatherings, outbreaks in places like assisted living facilities and workplace transmission.
In an appeal to residents, Chau, the county's acting health officer, urged use of face coverings.
"This is not about you," Chau said this week, according to CNN affiliate KABC. "This is about our children. This is about our next generation ... I can appeal to them that you need to do this. You need to do this little sacrifice because we have to ensure that our environment is safe for our children to grow."
County health officials will review waivers submitted by schools that want to reopen for in-person classes, KABC reported. In their applications, the schools must be able to show they consulted with parent and labor groups and include their reopening plans, according to the affiliate.