CHICAGO (CBS) — A second wave of flu is hitting the area, and the culprit is ineffective flu shots, doctors say.

At Elmhurst Hospital, doctors are seeing an unusual uptick in cases, especially this late in the season.

The influenza A strain that arrived last fall has been succeeded by an influenza B strain. This year’s vaccine has been only 36 percent effective, experts say.

“This year it wasn’t as effective. People who thought they were protected were not,” said Dr. Karl Vos of Elmhurst Hospital

The latest data from the Illinois Department Of Health shows 39 people with the flu were in intensive care last week. The Chicago and Champaign regions had the highest number of cases.

Doctors warn the B strain is powerful. Many who show up at the emergency room wind up getting admitted, especially older patients with other health issues, such as asthma and COPD.

State health officials say a rabid increase in cases at the end of 2017, the highest in eight years. And now, they are seeing flu activity much later in the season than normal.

Doctors say it is not too late to get a flu shot. If your local provider or physician has run out, hospitals are a better bet.