Half-Dose Flu Shots Work Best in Women

Vaccine Shortage? Half-Dose Flu Shots OK for Healthy, Young, Female Adults

Medically Reviewed by Louise Chang, MD on December 08, 2008
From the WebMD Archives

Dec. 8, 2008 -- 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.

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Engler, R.J.M. Archives of Internal Medicine, Dec. 8/22, 2008; vol 168: pp 2405-2414.

Falsey, A.R. Archives of Internal Medicine, Dec. 8/22, 2008; vol 168: pp 2402-2403.

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