It’s an unexpected phenomenon that public health experts are still trying to decode.

The levels of influenza-like illnesses being reported now are as high as the peak of the swine flu epidemic in 2009, and exceed the last severe seasonal flu outbreak in 2003 when a new strain started circulating, said Anne Schuchat, the U.S. Centers for Disease Control and Prevention’s acting director. Swine flu, which swept the globe in 2009 and 2010, sickened 60.8 million Americans, hospitalized 274,304 and killed 12,469, according to CDC data. Deaths from the current outbreak will likely far outstrip those of the 2009-2010 season.

“This is a difficult season, and we can’t predict how much longer the severe season will last,” she said. “I wish there was better news, but everything we are looking at is bad news.”

The primary type of influenza this year hasn’t changed enough from previous seasons to be considered a novel strain, Schuchat said. The agency’s virologists are studying it to determine if there are any other explanations for why it’s been so hard-hitting.

“We have a lot to learn still about influenza,” she said. “It’s a wake-up call about how severe influenza can be, and why we can never let down our guard.”

Deaths from influenza and pneumonia, which are closely tied to each other in the winter months, were responsible for 1 of every 10 deaths last week, and that’s likely to rise, Schuchat said in a conference call Friday. There were 40,414 deaths in the U.S. during the third week of 2018, the most recent data available, and 4,064 were from pneumonia or influenza, according to the CDC data. The number for that week is expected to rise more reports are sent to the agency.

It gets worse. The death toll in future weeks is expected to grow even higher because flu activity is still rising—and the number of deaths follow the flu activity. Hospitalization rates are already approaching total numbers seen at the end of the flu season, which may not be for months.

“Unfortunately, more deaths are likely to happen,” Schuchat said. “Over the next few weeks, we do expect and it would make sense to see more pneumonia and influenza-related deaths. The people who are likely to die are already in the hospital.”

This year’s flu could be more calamitous than outbreaks going back decades, but there’s no way to know for sure. It’s difficult to compare the severity of influenza across seasons for more than a handful of years because of changes in how the virus is handled in the U.S. The CDC started recommending universal vaccination to stop the spread of the virus in 2010, after previously targeting only those in high-risk groups who were most likely to die from an infection.

The agency reported another 10 deaths among children this season, bringing the total to 63 so far. Half had no other medical conditions that would place them in the high-risk category, and only about 20 percent were vaccinated.

The agency only started counting deaths among children in 2004, after a particularly severe season. That year, the number of doctor’s office visits for the flu peaked at 7.6 percent; last week it was 7.7 percent.