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Cold, Flu, & Cough Health Center

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Seasonal Flu Shot Some Help vs. Swine Flu?

Mexican Study: Some H1N1Swine Flu Protection in Seasonal Shot; U.S. Data Show No Support for This Conclusion
WebMD Health News
Reviewed by Louise Chang, MD

Oct. 6, 2009 - A small study of Mexican H1N1 swine flu patients suggests that seasonal flu shots might offer some protection against the new flu. But CDC data show no hint of such protection.

The study, which looked at patients of a small respiratory disease hospital in Mexico City, showed that those who received a flu vaccine during the 2008-2009 flu season were 73% less likely to have been infected with H1N1 swine flu than unvaccinated patients.

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Moreover, the study suggested that seasonal flu vaccine might make H1N1 swine flu less severe. There were no deaths among the eight vaccinated patients who came to the hospital with lab-confirmed H1N1 swine flu. But among 52 unvaccinated patients with H1N1 swine flu, 18 died.

Despite the findings, which they call "preliminary," study researchers Lourdes Garcia-Garcia, MD, of Mexico's National Institute of Public Health, and colleagues, warn that seasonal flu vaccination is not sufficient protection against H1N1 swine flu.

"Notwithstanding this contribution to protection, a specific vaccine against A/H1N1 2009 [swine flu] is crucial," they conclude in their report, published in the advance online issue of BMJ.

The Mexican data stand in stark contrast to U.S. data and Australian data, which show no hint that seasonal flu vaccination has any effect on the current H1N1 swine flu.

An unpublished study from Canada reportedly showed just the opposite of the Mexican study -- that seasonal flu vaccine might make people more vulnerable to H1N1 swine flu. U.S. and Australian data offer no support for this hypothesis, either.

It's "biologically conceivable" that seasonal flu vaccine might offer some protection against H1N1 swine flu, says flu expert John Treanor, MD, chief of infectious diseases at the University of Rochester, N.Y.

"We would not predict the protection would be very robust," Treanor tells WebMD. "This does not suggest in any way that we don't need a vaccine for H1N1 [swine flu]."

There are CDC data suggesting that older people -- possibly due to exposure to a seasonal H1N1 virus that circulated before 1957 -- may have some slight protection against H1N1 swine flu.

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