"Overall, those that received a seasonal flu vaccine last year had a 45% less chance of developing swine flu compared to those who did not get the seasonal flu vaccine," says study senior author, retired U.S. Army Col. Jose Sanchez, the influenza team leader at the Armed Forces Health Surveillance Center in Silver Spring, Md.

However, the Armed Forces Health Surveillance Center study suggests the 2008-2009 seasonal flu vaccine protects certain age groups from H1N1— the oldest and youngest — while offering no protection at all for others.

Health experts this year have long stated that the best protection against swine flu is the swine flu vaccine. The current seasonal flu vaccine is not believed to protect against H1N1.

The military study monitored the spread of H1N1 in the military community between April and Oct. 15th of this year, and included active duty personnel. It included data from health files of 1,205 people who had contracted H1N1 as well as 4,820 controls who did not come down with swine flu, Sanchez says.

Anyone who had received this year's seasonal flu vaccine or H1N1 vaccines were excluded, Sanchez says.

According to the military data, if you received a seasonal flu vaccine last year, you would be at a 62% decreased chance of being hospitalized with swine flu and a 42% decreased risk of visiting a doctor, Sanchez says.

"Why it protected against more severe hospitalized type outcomes — in what way or shape — we don't' know," he says.

The author says scientists thought they would see an increasing trend in protection from younger to oldest, but that was not the case. In the 40 and older group, there was a 55% lesser risk getting swine flu. In 17 to 24 group there was a 50% lowered risk of contracting H1N1.

But in the 25 to 39 year-old group there was no protection at all, Sanchez says.

Why? Though the study doesn't explain why young adults received no protection, Sanchez says viruses similar to H1N1 were not circulating during the childhoods of those in this age range, but was documented during the younger years of the other two groups.

"This study to a certain extent makes a lot of sense," says infectious disease physician Neil Fishman, director of health care epidemiology and infection control at the University of Pennsylvania Health System. "It doesn't look like it prevents infection, but looks like the (seasonal) vaccine helps prevent against severe disease."

The seasonal vaccine has contained an H1N1 strain since the late '70s, Fishman says. "Although it is very different than the novel 2009 H1N1 strain circulating now, it's not that surprising to me that it might protect somewhat against disease."

Gregory Gray, professor in the epidemiology department at the College of Public Health at the University of Iowa, and the director of the Center for Emerging Infectious Diseases there, says the military population, large groups of people in close quarters, is a good one to use to study respiratory diseases. "I'd have a tendency to very much believe these data," Gray says.