Previous seasonal influenza infections may provide partial immunity to H1N1

The H1N1 virus (photo: Centers for Disease Control and Prevention)

Previous seasonal influenza infections may provide partial immunity to H1N1, according to a new study. This information could alleviate some of the fears brought on by swine flu and the low availability of H1N1 vaccine.

"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., a 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 H1N1 immunity study
The researchers used a large flu database funded by the National Institutes of Health, as well as blood samples from adults representing the general 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. "We found that the immune system's T cells can recognize a significant percent of the markers in swine flu."

T cells, the infection-fighting white blood cells in the immune system, recognize and attack viruses they have seen before. The study found 69% of T cells recognized molecular pieces of the H1N1 virus.

"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," said Dr. Peters.

Is the H1N1 vaccine still needed?
T cells are not the only part of the immune system involved in fighting H1N1. The researchers also looked at the response of B cells, of which only 17% recognized the H1N1 virus.

T cells “recognize the virus inside cells, so they are responsible for clearing the infection once you have it," explained Dr. Peters. B cells “produce antibodies in the bloodstream and try to find the virus before it ends up in cells, so they are what prevents the disease.”

"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," warned Dr. Peters. "From our findings, we see that it is necessary to get a shot. Yet it provides an explanation why you do not have to be absolutely concerned if you have not been able to get a shot yet."

The study partially explains why young people are more severely affected by swine flu, as an H1N1 virus has not been the dominant strain during flu season for a few decades.