May 21 (HealthDay News) -- 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, a U.S. infectious-disease expert said Wednesday.
"The further back you go in time, the more likely you are to have been exposed to H1N1 virus back before 1957, and there is a possibility that having exposure to that virus many years ago may allow you to have some [antibody] reaction to the new H1N1 that's now circulating," Dr. Daniel Jernigan, deputy director of CDC's Influenza Division, said during a teleconference.
That may explain why the new swine flu outbreak is striking a disproportionately large number of children and young adults. The regular seasonal flu typically takes the biggest toll among the very young and the elderly.
The current H1N1 virus is a genetic cousin of the more virulent H1N1 "Spanish flu" virus of 1918 that killed 20 million to 50 million people worldwide, and up to 500,000 in the United states. Seasonal versions of this virus circulated throughout the United States until it was replaced in 1957 by the H2N2 "Asian flu" pandemic virus, which caused 70,000 deaths in the United States.
In 1977, the H1N1 "Russian flu" virus emerged, but people exposed to H1N1 before 1957 were largely immune to this strain, according to the U.S. National Institute of Allergy and Infectious Diseases.
Jernigan said studies have found evidence of H1N1 antibody activity in blood from older people. "We can infer from that, to some degree, that there is some level of protection," he said.
But he added a note of caution, saying that many years have passed and the new virus, although the same subtype, is different from the H1N1 seasonal flu virus circulating before 1957.
Jernigan also disagreed with an assessment Tuesday by the World Health Organization's director-general, Dr. Margaret Chan, who said production of an H1N1 swine flu vaccine could not begin until mid-July at the earliest, weeks later than previous estimates.
Chan's reasoning: swine flu virus isn't growing very fast in laboratories, making it hard for scientists to get the key ingredient they need for a vaccine -- the "seed stock" from the virus. And she said it would then take months before a vaccine would be available, the Associated Press reported.
However, Jernigan said that in the United States, the steps needed to produce a swine flu vaccine are moving ahead on schedule, and a vaccine should be ready by the fall. "We are moving along and have not had significant delays here in the U.S. with the development of the vaccine candidates," he said.
Jernigan noted that creating a vaccine for the new H1N1 flu is a complicated process. He also suggested making the regular, seasonal flu vaccine available earlier to Americans than its usual late September or early October introduction.
"At this point when to vaccinate [with the new H1N1 vaccine] is going to be driven largely by when it's available," Jernigan said. "If possible we want to have an earlier roll-out of the seasonal influenza vaccine, to make it easier for an additional vaccine if that's the ultimate policy," he said.
"For that reason we will be working closely with manufacturers and with multiple advisory committees through the federal government and with other partners to make sure that what is recommended in terms of timing is feasible and can be initiated, to offer the most protection to the most folks," Jernigan said.
In the United States, most cases of swine flu continue to be no worse than seasonal flu, health officials said. Testing has also found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza, according to the CDC.
The new swine flu is a highly unusual mix of swine, bird and human flu viruses. Experts worry that, if the new flu virus mutates, people would have limited immunity to fight the infection.
The CDC is concerned with what will happen as the H1N1 virus moves into the Southern Hemisphere, where the flu season is about to start. The agency is also preparing for the virus' likely return in the fall to the Northern Hemisphere.
On Tuesday, U.S. health officials said that people hospitalized with the swine flu who have underlying health problems fare worse than otherwise healthy people who also have been hospitalized. This buttresses the belief that the swine flu is no more dangerous than regular flu, the officials said.
On Wednesday, the CDC was reporting 5,711 U.S. cases of swine flu in 48 states, including 10 deaths. The three latest deaths include a 44-year-old man in Missouri, the state's first such fatality, a 57-year-old woman in Arizona, that state's second confirmed swine flu death, and a 21-year-old man in Utah who was overweight and had underlying medical conditions, according to the AP.
The World Health Organization on Thursday was reporting 11,034 diagnosed cases in 41 countries, including 85 deaths, mostly in Mexico, believed to be the source of the outbreak.
SOURCES: May 20, 2009 teleconference with Daniel Jernigan, M.D., Ph.D., medical epidemiology, Influenza Division, U.S. Centers for Disease Control and Prevention; May 19, 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