(CNN) So far this season,105 children have died from the flu, according to data released Friday by the US Centers for Disease Control and Prevention. This is the highest number of child flu deaths at this point in the season since the CDC started keeping records in 2004, except for the 2009 flu pandemic.

It has been an "unusual" flu season with a higher proportion of children and young adults affected than the older population, according to Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center.

The earlier prevalence of influenza B -- a flu strain that tends to be more common in children -- could be a reason why more children were affected, Schaffner said. Also, as the number of influenza B cases decreased, the number of H1N1 cases increased, he said. H1N1 is a subtype of the influenza A strain, which also affects children more than adults.

"This is the first time in 25 years where [influenza B] became so common so early," said Dr. Buddy Creech, an associate professor of pediatric infectious diseases at Vanderbilt University Medical Center.

Neither Schaffner nor Creech know why this year's influenza timeline is so different.