WASHINGTON (Reuters) - The 2004-2005 U.S. flu season was moderate, but the virus killed at least 36 children, the U.S. Centers for Disease Control and Prevention said on Thursday.

On the same day, CDC vaccine advisers issued new recommendations aimed at preventing healthcare workers from spreading the virus, and a government investigator again criticized the U.S. government´s preparations for a major influenza outbreak.

Influenza hits every year, killing about 36,000 Americans and up to 500,000 people globally in an average season. These numbers could rise to hundreds of thousands in the United States alone and millions globally if a serious pandemic occurred -- something most experts expect eventually.

This year, the CDC said, flu season peaked in February and was at levels considered epidemic from Feb. 14 to April 9.

An average of 15 percent of all suspected influenza cases reported to the CDC by hospitals actually turned out to be flu, the agency said. The rest were either something else or unidentifiable.

Health officials have tried to encourage annual vaccination against flu, but their efforts fell apart during the last season when Chiron Corp. lost its license to make vaccine and had to destroy half the anticipated U.S. supply.

The CDC´s Advisory Committee on Immunization Practices recommended on Thursday that hospitals and clinics require every healthcare worker to get flu vaccinations or sign a form saying they declined it.

About 36 percent of healthcare workers get flu shots, the according to surveys, even though they are very likely to be infected and can pass the virus onto vulnerable patients before they show symptoms.

On Thursday, Congress heard criticism from a    Government Accountability Office expert, Marcia Crosse, who said the CDC and the  Department of Health and Human Services issued "mixed messages" about who should get vaccinated.

Despite the shortage and long lines at the beginning of flu season in October, vaccine makers including Sanofi-Aventis and MedImmune threw out some doses in the end. The flu vaccine must be reformulated every year to keep up with the rapidly changing virus.

Crosse told the House Committee on Government Reform that an influenza pandemic could strain the available capacity of hospitals.

"Public health officials we spoke with said that a large-scale outbreak, such as an influenza pandemic, could strain the available capacity of hospitals by requiring entire hospital sections, along with their staff, to be used as isolation facilities," she said.