Although this flu season is shaping up to be milder than last season, health officials are still worried about the rate of potentially fatal influenza-related bacterial infections.

Each year, between 5% and 20% of the U.S. population come down with the flu, according to the Centers for Disease Control and Prevention. So far this season, roughly 2.4% of the population has sought treatment for influenza. The most common strain of the flu making the rounds this year is influenza A, or the H1 strain, which health officials included in this year’s batch of vaccines, CDC officials reported.

But flu-related bacterial infections, such as methicillin-resistant Staphylococcus aureus (MRSA) and bacterial pneumonia, are still prevalent this season, giving health officials some cause for concern. Many of the flu-related deaths every year stem from these types of bacterial infections. Each year, 36,000 people die from complications due to the flu, 90% of whom are seniors, according to CDC data.