Half-dose flu shots may be just as effective as a full dose in people under age 50 who have been previously vaccinated, a U.S. study has found.

The findings — published Monday in Archives of Internal Medicine — suggest that in times of vaccine shortage, public health officials may consider giving only half doses to healthy young adults.

Dr. Renata Engler of the Walter Reed Army Medical Center in Washington, D.C., and colleagues conducted a clinical trial involving healthy adults aged 18 to 64.

Between November and December 2004, 554 adults received a full dose of influenza vaccine and 556 received a half-dose. All had been vaccinated within the past one to three years.

The researchers collected blood samples before the vaccination and 21 days after, testing for antibodies against influenza. Participants also recorded any symptoms they experienced during this time period.

Antibody responses in healthy, previously immunized adults "were not substantially inferior to the full-dose vaccine, particularly for ages 18 to 49 years," the researchers write.

No difference seen

In addition, from November 2004 through March 2005, rates of medical visits for respiratory or cardiovascular reasons were no different between those vaccinated with a full dose or with a half-dose.

"Reduced dosing could have a significant impact on the response to vaccine shortages, particularly at a local level when faced with considerable delays in vaccine supply delivery," the researchers write.

Because half-doses were associated with fewer side-effects, reducing dosage could also make vaccination more acceptable, particularly among groups of people who experience more adverse effects.

The researchers also found that women of all ages had a greater response to both doses of the vaccine than men.

In fact, women receiving a half-dose of the vaccine had similar antibody responses to men receiving a full dose, suggesting guidelines for vaccine use during shortages should take sex, as well as age, into consideration.

"In trying to stretch vaccine supplies to protect the largest number of persons, we must use care to not waste vaccine by using low doses, which will ultimately not be protective," Dr. Ann Falsey of New York State's Rochester General Hospital writes in an accompanying editorial.

"This study clearly shows that half-dose trivalent inactivated vaccine in young healthy women is a rational way to extend vaccine supply in times of critical shortage."