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.
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An unpublished study from Canada reportedly showed just the opposite of the Mexican study -- that seasonal flu vaccine might make people more vulnerable to H1N1 swine flu.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.
And the fact that adults become protected against H1N1 swine flu after just a single dose of the new H1N1 swine flu vaccine suggests some kind of priming, either exposure to or vaccination against seasonal H1N1 virus.
Treanor suggests that this is because the current flu pandemic is something different than scientists have ever before seen. It's certainly not like a seasonal flu, but it's also not quite like the more severe flu pandemic that had been feared.
"We are learning it is possible to have something between a full blown pandemic and just a bad season," Treanor says. "You see this high transmissibility with relatively low mortality. It's not what you might see with a totally new [human flu virus] and it's not what you see in a bad flu season."