* Biggest impact of pandemic seen in emergency rooms

* Vaccine shortages may add to strain

By Julie Steenhuysen

CHICAGO, Nov 5 (Reuters) - Last May, an influx of children with flu symptoms and their worried families flooded the emergency department of Children's Memorial Hospital in Chicago, swelling the normal patient load from 150 a day to 400 and leaving a net loss for the month of $325,000.

Patient volumes at the not-for-profit hospital are beginning to rise once again as a second wave of the H1N1 flu pandemic -- which has killed 1,000 Americans and more than 5,700 people globally -- works its way through the Northern Hemisphere.

"At this point in the year, we would expect to see around 180 patients a day. We're seeing around 250 patients a day," Donald Camp, an administrator at the hospital, said.

"We are watching our volumes closely, but to date we have weathered this all right," Camp said in a phone interview.

Wall Street analysts and academic experts say so far the H1N1 pandemic has had a mild impact on hospitals in the United States, and will likely fall far short of the most dire predictions of hospitals chock full of patients admitted with severe cases of flu.

Dr. Eric Toner of Center for Biosecurity at the University of Pittsburgh Medical Center said a severe flu pandemic like the one in 1918 could cost U.S. hospitals $3.9 billion, but he expects this pandemic to have far less impact.