At one high school in San Diego, more than 1,000 of the school's 2,600 students were absent on Wednesday, and according to school officials, it's due to possible flu cases. Several students tested positive for the flu and negative for COVID-19.
Experts say masking and other measures to curb the spread of COVID have prevented people from being exposed to the flu, leading to weakened immunity.
Outpatient visits for influenza-like illnesses have particularly risen for children ages 4 and younger to more than 120,000 during the week ending Oct. 1, a 25.5% spike from the roughly 95,600 visits for this age group that were seen the week ending Aug. 6.
The CDC has previously warned the U.S. may see a harsh flu season after few to no cases were reported over the last two years.
ABC News contributed to this report.