TUESDAY, Oct. 6, 2009 (HealthDay News) -- The seasonal flu vaccine may offer partial protection against the pandemic H1N1 swine flu, but not enough to prevent a person from catching the swine flu, Mexican researchers say.
In a study of hospital patients during the H1N1 epidemic in Mexico City last spring, the researchers found that those who had had a seasonal flu shot and were infected with the H1N1 flu had significantly milder cases than those who had not received a seasonal flu shot.
For the study, a group led by Dr. Jose Luis Valdespino-Gomez reviewed the outcomes of 60 patients with swine flu and 180 patients with other diseases. Some of the patients reported having received a 2008-09 seasonal flu shot.
"Seasonal vaccine may provide partial protection against pandemic influenza, particularly severe forms of the disease," said Valdespino-Gomez, an epidemiologist at Laboratorios de Biologicos y Reactivos de Mexico.
But Valdespino-Gomez cautioned that these findings do not mean that a seasonal flu shot will fully protect you from the H1N1 swine flu or its complications. "These results do not mean that people should not be vaccinated with the H1N1 influenza vaccine," he said. "The recommendation is to receive both seasonal and H1N1 vaccines."
This year's seasonal flu vaccine is available now, and the new H1N1 vaccine is about to be distributed. People should get both shots, according to the U.S. Centers for Disease Control and Prevention.
The seasonal vaccine may boost existing antibodies in those who have had a similar flu virus or a vaccination against seasonal flu, the researchers say.
The study, published in the Oct. 7 online edition of the British Medical Journal, has limitations because of the small number of participants. "In addition, the control group had a high frequency of chronic underlying conditions and a high frequency of seasonal vaccination, which may underestimate our results," Valdespino-Gomez said.
Dr. Marc Siegel, an associate professor of medicine at New York University School of Medicine in New York City, isn't surprised to hear that a seasonal flu shot offers some protection from the swine flu.
"The implication is that if you trigger an antibody response to an H1N1 flu virus of any kind, it may be useful as part of the body's attempt to fight the flu," he said. "That would explain why it wouldn't immunize you, but could prevent you from having a severe outcome."
Siegel cautioned that the seasonal vaccine the study participants received was last year's vaccine. This year's seasonal vaccine is a different mix of viruses so it may not have any protection against the H1N1 swine flu, he said.
"People who have gotten a seasonal flu shot should not assume they are protected from the H1N1 flu," Siegel said. "Clearly, the amount of protection you are getting from a seasonal flu shot is not nearly of the magnitude that would take the place of getting this year's H1N1 vaccine."
You need both shots, he said.
For more information on flu, visit the Flu.gov.