Whole new flu vaccine needed next year: WHO

A nurse injects flu vaccine into a flood victim at a shelter in Villahermosa November 7, 2007. The three most common influenza viruses circulating globally have changed significantly and next year's flu vaccines should be updated, the World Health Organization recommended.

WASHINGTON (Reuters) - The three most common influenza viruses circulating globally have changed significantly and next year's flu vaccines should be updated, the World Health Organization recommended.

A flu epidemic is well under way in the United States, and the U.S. Centers for Disease Control and Prevention said it had clear indications that the viruses have mutated, making the current vaccine less effective.

WHO said similar reports were coming in globally and it appears next year's vaccine will have to be completely new.

"Forty-four states reported widespread influenza activity," the CDC said in its weekly flu report on Friday. Ten American children have died of flu this season, the CDC said.

About one third of people tested for flu because of their symptoms actually had influenza, the CDC said. Other viruses cause similar symptoms to flu, marked by a sudden onset of fever, muscle aches and weakness.

Influenza viruses mutate constantly. Because of this, experts meet throughout the year to monitor the flu seasons and consider how to formulate the vaccine.

Flu vaccines contain the three most common circulating strains, usually two "A" strains and one "B" strain.

"As in previous years, national control authorities should approve the specific vaccine viruses used in each country," the WHO said.

Even if the flu vaccine is not a perfect match, it can protect people against the most serious effects of the infection. In an average year flu kills between 250,000 and 500,000 people globally.

Four antiviral drugs can help treat a case of the flu, but the WHO said H1N1 viruses in 18 countries had developed mutations that made them resist the effects of the best drug, Tamiflu, or oseltamivir, made by Roche AG and Gilead Sciences.

Resistance to the older drugs, amantadine and rimantadine, is now so widespread that their use is no longer recommended.

Companies usually begin the process of making new flu vaccines several months before the new flu season starts. Flu vaccines are made using a time-consuming process that involves chicken eggs.

WHO starts the process of helping advise companies -- and the governments that buy their vaccines -- on which three flu strains would best protect the most people in the upcoming flu season. (Reporting by Maggie Fox; Editing by Will Dunham)