WEDNESDAY, May 27, 2009 (HealthDay News) -- As the H1N1 swine flu virus continues to wax and wane in different parts of the country, U.S. health officials said they were working as fast as possible to learn as much as they can about the novel pathogen before the return of the flu season in the fall.
The reason for the urgency: Some past pandemics were preceded by "herald waves" of a flu strain that surfaced at the end of one flu season, only to return with far greater consequences the next flu season.
"We are mindful that pandemics of influenza have sometimes come in waves," Dr. Anne Schuchat, the U.S. Centers for Disease Control and Prevention's interim deputy director for science and public health program, said during a Tuesday afternoon news conference. "The very severe 1918 pandemic had a moderate herald wave in the spring and a much more severe second wave in the fall. So that very terrible experience of 1918 is in our minds."
Some estimates have placed the worldwide death toll from the 1918 outbreak -- often referred to as the "Spanish Flu" -- as high as 40 million people.
"We are really on a fast track, over the next to eight to 10 weeks, to learn as much as we can as this virus heads to the Southern Hemisphere [where flu season is just beginning] and to strengthen our planning for the surge of illness that we expect to experience here in the fall," Schuchat added.
Scientists will be looking to see if the H1N1 swine flu virus mutates or becomes resistant to antiviral medications, or is more easily spread among people, she said.
Schuchat said there's no way to tell now if the H1N1 virus will be more virulent when -- and if -- it returns to the Northern Hemisphere with the approach of winter. "Whether it will dominate among the seasonal flu viruses or whether it will disappear is not predictable right now," she said.
To date there have been 7,927 confirmed and probable cases of infection in the United States, the CDC reported Wednesday. Most of the cases have been mild and patients have recovered quickly.
The CDC was reporting 11 deaths linked to the swine flu, and all of the victims had underlying health problems before they were infected.
The World Health Organization said Wednesday that 48 countries have reported 13,398 cases of infection, including 95 deaths, most of them in Mexico, where the outbreak began.
The CDC said last week that progress was being made toward the development of an H1N1 swine flu vaccine, with two promising candidate viruses for use in such a shot. And U.S. Department of Health and Human Services Secretary Kathleen Sebelius said Friday that the federal government was allocating $1 billion to the search for a swine flu vaccine.
In the United States, most cases of the swine flu continue to be no worse than seasonal flu. Testing has found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza, according to the CDC.
The CDC says some older people may have partial immunity to the H1N1 swine flu virus because of possible exposure to another H1N1 flu strain circulating prior to 1957. So far, 64 percent of cases of swine flu infection in the United States have been among people aged 5 to 24, while only 1 percent involves people over 65, officials said last week.
For more on swine flu, visit the U.S. Centers for Disease Control and Prevention.