May 26 (HealthDay News) -- Progress has been made toward developing a viable H1N1 swine flu vaccine, with experts at the U.S. Centers for Disease Control and Prevention saying they have two promising candidate viruses for use in such a shot.
Meanwhile, a Chicago man and a 50-year-old New York City woman died of swine flu over the holiday weekend, bringing the national toll to 12 in the United States, the Associated Press reported.
But good news came Friday from CDC officials, who reported that they are closer to a viable vaccine for this new strain of flu.
"Today CDC received, from one institution, a candidate vaccine virus," Dr. Anne Schuchat, the CDC's interim deputy director for science and public health program, said during a news conference on Friday.
The strain was created by "combining the genes of the novel H1N1 virus with other parts from other viruses," Schuchat explained. This type of hybrid virus will grow more easily in eggs -- an essential part of the vaccine production process.
The CDC, along with the U.S. Food and Drug Administration, has also created a second candidate virus using reverse genetics, Schuchat added.
The CDC is testing both viruses to make sure they can stimulate an optimal immune response, Schuchat said. "After that work is done, suitable viruses will be sent out to manufacturers. We expect by the end of May that will happen," she added.
Also Friday, Department of Health and Human Services Secretary Kathleen Sebelius announced that the federal government was allocating $1 billion to the search for a swine flu vaccine, the AP reported. The funding is aimed at pilot testing of a vaccine and the setting up of a "pre-pandemic" stockpile that HHS said would cover at least 20 million people, including health-care workers and people at high risk for complications from the illness.
In related news, a study released on Friday suggests that many of the genes that make up the new H1N1 swine flu virus have been circulating undetected in pigs for more than a decade.
Scientists at the CDC and elsewhere sequenced the genomes of dozens of samples of the swine flu strain and found it is distantly connected to its closest viral relatives.
The researchers also found that the new H1N1 strain lacks genes that -- in other influenza A strains -- confer ease of transmission and virulence.
CDC officials discussed the findings at a teleconference Friday. The findings were released early in the journal Science because of the broad interest in this new strain of swine flu.
"From our analysis, we have confirmed that the novel H1N1 virus likely originated from pigs, based on data that each of the genetic components of this virus are most closely related to corresponding influenza virus genes identified from swine influenza viruses," said Dr. Nancy Cox, chief of CDC's Influenza Division.
However, this new virus is not similar to seasonal H1N1 viruses, she said. In their analysis of 70 samples of the new H1N1 virus from the United States and Mexico, the researchers found minor genetic differences, but consider the virus to be basically homogeneous, Cox noted.
Knowing the genetic makeup of the virus makes it easier to come up with a candidate vaccine, Cox said. "We see much less variation among these new H1N1 viruses than we do for typical seasonal influenza viruses," she said.
Sequencing the virus' genetic code is also important for planning the public health response, including knowing which antiviral medications will be effective and which won't, Cox said.
And, Cox added, "We can take measures to be sure that the virus doesn't reemerge in a slightly different form."
In the future, scientists will need to keep a closer eye on pig populations to spot similar emerging flu viruses, the researchers said in the Science paper.
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.
While the new swine flu only seems to cause relatively mild infection, experts worry that, if the virus mutates, people would have limited immunity to it. The CDC is concerned that, as the H1N1 virus moves into the Southern Hemisphere, where the flu season is just getting under way, it could mutate and return in a more virulent form in the Northern Hemisphere next fall.
On Thursday, U.S. health officials said that, while many states are still reporting new cases of infection, there seems to be an overall decline in visits to doctors and hospitals by people with the disease, indicating that the outbreak might be subsiding.
The CDC reported on Wednesday that some older people may have partial immunity to the new 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.
As of Monday, the World Health Organization was reporting 12,500 diagnosed swine flu cases in 46 countries, including 91 deaths, according to the AP.
SOURCES: May 22, 2009, teleconference with Nancy Cox, Ph.D., chief of U.S. Centers for Disease Control and Prevention's Influenza Division; May 21, 2009, teleconference with Anne Schuchat, M.D., interim deputy director for science and public health program, U.S. Centers for Disease Control and Prevention; Associated Press