Swine Flu Could Eventually Affect 40% of Americans: CDC

July 25 -- FRIDAY, July 24 (HealthDay News) -- The H1N1 swine flu could end up affecting as many as 40 percent of Americans, if one includes workers who stay home to care for people who contract the illness, U.S. health officials said Friday.

The projection from the U.S. Centers for Disease Control and Prevention is based on the influenza pandemic in 1957, when almost 70,000 people in the United States died from the flu.

"Our planning assumptions for a severe pandemic were that up to 40 percent of the workforce might be affected and not able to work, either because they were ill or because they needed to stay home to care for an ill family member," Dr. Anne Schuchat, director of CDC's National Center for Immunization and Respiratory Diseases, said Friday in a press conference.

But even if the new H1N1 virus never reaches that proportion, it is expected to gain strength come fall.

"We had a 6 to 8 percent attack rate just during the spring months," Schuchat said. "We think that in a longer winter season, attack rates would be two to three times as high as that," she said.

A public health campaign and a vaccination program, which will probably begin in October, could reduce the impact of the H1N1 swine flu, she said.

"We think we can limit, somewhat, the illness and severe complications of that kind of virus circulation with updated guidance and, of course, with the efforts we are making towards the development of a vaccine," Schuchat said.

Vaccine trials, already underway in Australia, are expected to begin in the United States next week, Schuchat said.

U.S. officials hope to have 160 million doses of injectable swine flu vaccine on hand by October, with more doses coming in the form of a nasal spray -- if trials of experimental vaccines are successful.

To determine who should receive the vaccine first, the CDC's Advisory Committee on Immunization Practices will meet Wednesday.

In the Southern Hemisphere, where it is winter now, seasonal flu and the new H1N1 swine flu continue to spread, Schuchat said.