More U.S. children have died this flu season than at any time since the swine flu pandemic 15 years ago.

The flu vaccination rate for U.S. children has plummeted from about 64% five years ago to 49% this season.

“This number that we have now is almost certainly an undercount, and one that — when the season is declared over, and they compile all the data — it’s almost certain to go up,” said Dr. Sean O’Leary of the American Academy of Pediatrics.