WebMD Medical News
Daniel J. DeNoon
Louise Chang, MD
Nov. 20, 2009 - People who got last year's seasonal flu vaccine are at lower
risk of H1N1 swine flu illness -- particularly severe disease, a study of U.S.
military personnel shows.
Overall, getting a seasonal flu shot or sniff cut the risk of swine flu by
45%. It cut the risk of getting a normal case of swine flu by 42%, and
cut the risk of being hospitalized with swine flu by 62%.
Oddly, not everyone was protected. The vaccine was not effective in
personnel age 25 to 39. But it was 50% effective in those under age 25 and 55%
effective in those over age 39, find Jose Luis Sanchez, MD, MPH, influenza team
leader at the Armed Forces Health Surveillance Center in Silver Spring,
Md., and colleagues.
"This strongly suggests that prior vaccination with seasonal influenza
vaccine confers some degree of cross-immunity against H1N1 swine flu," Sanchez
tells WebMD. "It's probably not conferring protection against H1N1 infections,
but probably is protecting against disease and hospitalization once you are
How can this be? CDC studies show that people who got the seasonal flu
vaccine do not have antibodies in their blood that neutralize the H1N1 swine
But antibodies are only one arm of the immune system. Another arm is
cell-mediated immunity, in which T cells learn to recognize pathogens. The next
time they see these pathogens, these "memory" T cells marshall various defenses
that kill off infected cells to limit the spread of infection.
Just this week, Jason A. Greenbaum, PhD, of the La Jolla Institute for
Allergy and Immunology and colleagues, reported that T cells primed to
recognize seasonal H1N1 flu bugs trigger immune defenses when they see 2009
H1N1 swine flu bugs.
That, Sanchez says, is his best guess to explain why seasonal flu vaccines
protect against severe disease, but not against infection. But why aren't
people age 25 to 39 protected?
It may take at least two exposures -- either two seasonal flu shots given at
different times, or exposure to the flu plus one later flu shot -- to be primed
to fight off a new flu bug.
Because H1N1 viruses did not circulate from 1958 through 1978, it's possible
that people born around that time had less chance to be primed by H1N1
infection during childhood, when people are most likely to catch the flu. The
years don't exactly match, but Sanchez finds the coincidence quite
"So now, 30 years later, you are 32, 35, and get the seasonal vaccine and
whoa! It didn't protect you against the H1N1 swine flu, because you were not
primed by an H1N1-like strain," he suggests.
What all this means, Sanchez says, is that getting the flu vaccine every
year offers extra benefits.
"The more vaccination or natural infection you get that allows your immune
system to be primed against different kinds of flu viruses, the better off you
will be," he says. "So this speaks for getting your seasonal flu vaccine. If
there's one message I want to get out, this is it."
Even so, it would be a bad idea to count on the seasonal flu vaccine to
protect you against H1N1 swine flu. The new vaccine raises antibodies that can
protect you from infection -- which would be a much better thing than getting
even a "mild" case of flu. And if you're also primed with a prior flu shot, so
much the better.
The researchers compared 1,205 lab-confirmed cases of 2009 H1N1 flu in
people of all branches of military service people to 4,810 service people
Sanchez presented the findings at the annual meeting of the American Society
of Tropical Medicine and Hygiene, held Nov. 18-22 in Washington, D.C.
SOURCES:Annual meeting of the American Society of Tropical Medicine and Hygiene,
Washington, D.C., Nov. 18-22, 2009.Jose Luis Sanchez, MD, MPH, influenza team leader, Armed Forces Health
Surveillance Center, Silver Spring, Md.Greenbaum, J. PNAS Early Edition.
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