Half-Dose Flu Shots Work Best in Women
In an emergency, how far could we stretch a short supply of
flu vaccine ?
That was a big question as recently as 2004, when a sudden U.S. shortage of
flu vaccine kept healthy adults from getting their
flu shots.
One solution considered in 2004 was to give healthy adults a half dose of
vaccine. An earlier study with the 2001/2002
flu vaccine suggested it might work.
As it turned out, the half-dose strategy wasn't used in the 2004/2005 flu
season. But it did happen as part of an experiment to see how well a half dose
would work. Because healthy adults were asked to hold off getting their flu
shots, it became ethical to give volunteers a half dose to see what
happened.
Renata J.M. Engler, MD, of Walter Reed Army Medical Center and colleagues
enrolled 1,114 volunteers. Half got the full-dose flu shot, and half got a half
dose. Neither the researchers nor the volunteers knew at the time who got which
shot.
There are three flu
types in the flu vaccine. For people aged 18-49, overall antibody responses
to half-dose vaccine were no worse than overall antibody responses to full-dose
vaccine.
Interestingly, 18- to 49-year-old women had better antibody responses than
did men for all three components of the flu vaccine.
For study participants aged 50-64, half-dose flu vaccine was as good as
full-dose vaccine for two of the three flu types (H3N2 and B) and not as good
for one type (H1N1).
Engler and colleagues conclude that if there's another flu shortage, it
might be a good idea to give half-dose vaccines to young adult women, and
perhaps to young adult men, but not to older men and women.
Did the half-dose vaccine actually keep anyone from getting the flu? That's
not known. People who got the half-dose vaccine didn't get any more
influenza-like illnesses than those getting the full dose -- but the study
ended before the 2004-2005 flu season was fully under way.
An editorial accompanying the study, Ann R. Falsey, MD, of the University of
Rochester, N.Y., ponders whether the half-dose strategy really stretches thin
supplies -- or whether it wastes precious vaccine.
"If more [flu] antibodies are better, how much is 'good enough' in a
time of critical vaccine shortage?" Falsey asks.
She notes that flu vaccine is still made the old-fashioned way -- in a
laborious process involving hens' eggs. A breakdown in this process is what led
to the 2004 flu vaccine shortage. And there's no guarantee such breakdowns
won't happen in the future.
Meanwhile, improved cell-culture techniques promise to deliver flu vaccines
that are faster to make and better matched for circulating flu strains than the
old hens' eggs technology.
"Perhaps the real message of this study is that better methods of
influenza vaccine production that are less prone to problems are clearly
needed," Falsey suggests. "The time has come to relegate the use of
eggs back to the kitchen where they belong."
The Engler study, and the Falsey editorial, appear in the Dec. 8/22 issue of
the Archives of Internal Medicine.
By Daniel DeNoon
Reviewed by Louise Chang
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