Previous Flu Infections May Provide Some H1N1 Immunity

Researchers at the La Jolla Institute for Allergy & Immunology have found that previous influenza infections may provide at least some level of immunity to the H1N1 "swine" flu. "The question we asked was, 'Is the swine flu more like the seasonal flu or like a totally new strain of influenza where there would be no immunity?,'" said Alessandro Sette, Ph.D., an internationally recognized vaccine expert and director of the La Jolla Institute's Center for Infectious Disease. "What we have found is that the swine flu has similarities to the seasonal flu, which appear to provide some level of pre-existing immunity. This suggests that it could make the disease less severe in the general population than originally feared."

The researchers used the Immune Epitope Database--a worldwide research tool developed and hosted by the La Jolla Institute and funded by the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health--to conduct their study. Initially, the research team conducted their studies using computer modeling, and later used blood samples of adults representative of the normal population.

"We looked at the molecular markers for seasonal influenza viruses dating back 20 years and compared them with the molecular markers of the H1N1 influenza virus," said Bjoern Peters, Ph.D., lead author on the study published in the Proceedings of the National Academy of Sciences. "We found that the immune system's T cells can recognize a significant percent of the markers in swine flu."

T cells are infection-fighting white blood cells in the body's immune system. "Nobody knows what level of immunity is sufficient for protection. We do know that a T cell response is not enough to prevent being infected by the virus. But, if infected, our data suggests that T cells in those who have previously been exposed to influenza may make the infection less severe," Peters said.

The findings are based on knowledge that the body's T cells recognize and will launch an attack against viruses--in this case certain molecular pieces of the swine flu--that they have seen before.

The research team also looked at the immune system's antibody-producing B cell response to the H1N1 virus. In this area, they saw only 17 percent recognition of the markers on the H1N1 as compared to seasonal flu. B cells, and their ability to produce antibodies that remember a virus, are the basis for vaccines to protect against subsequent infections by similar viruses. "Since the antibody recognition of the H1N1 virus was relatively low as compared to previous flu viruses, it means that the vaccine is important to prevent being infected by the H1N1 virus," Peters said.

"The data regarding pre-existing T cell immunity helps to explain why the general population seems to be faring better against the H1N1 virus than expected," Sette said. Nonetheless, H1N1 remains a serious concern as does seasonal influenza, he said.

According to CDC estimates, between 14 million and 34 million cases of H1N1 occurred between April and October 17, 2009, and between 63,000 and 153,000 H1N1-related hospitalizations during that same period. Between 2,500 and 6,000 H1N1-related deaths occurred during the same period, according to CDC estimates.

CDC officials have said up to 40 percent of the United States population may eventually be infected by the H1N1 virus. In a normal flu season, influenza infects as much as 20 percent of the United States population, causes more than 200,000 hospitalizations, and kills about 36,000 people.

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