OPTIONS VI: Inactivated Flu Vaccine Marches On in Army Study

TORONTO, June 20 -- The trivalent inactivated flu vaccine appeared to outperform the newer live attenuated vaccine in a large study of military personnel, a U.S. Army researcher reported here.

The difference in effectiveness was between 10% and 20% in each of three cohorts totaling nearly 700,000 active duty military personnel, according to Angelia Eick, Ph.D., of the Army Medical Surveillance Activity in Silver Spring, Md.

The study, covering the period from Sept. 1, 2005 through April 30, 2006, measured influenza-like illness, based on disease classification codes rather than lab-confirmed flu, Dr. Eick told attendees at the Options for the Control of Influenza meeting.

Although the live attenuated vaccine has been found to work better than the trivalent inactivated form in children, Dr. Eick and colleagues noted that there was limited data on the two vaccines in adults.

So, using the Defense Medical Surveillance System database, the researchers looked at medical records of soldiers ages 17 through 49 who were not pregnant and had received only one dose of either of the vaccines.

All told, 555,500 got the trivalent vaccine and 135,500 got the live vaccine, Dr. Eick said.

The study group was divided into three:

• A continuous cohort, in which military personnel could be placed no matter when they got their vaccination during the study period.
• A November cohort, which included only those who were vaccinated in November 2005.
• And an intense cohort, which included people who were vaccinated before the most intense period of the 2006 flu season, from Feb. 1 through March 31, as determined by the CDC.

In the first two cohorts, the outcome measure was all influenza-like illnesses that occurred more than 14 days after vaccination; in the intense cohort, it was the first such illness.

In the three cohorts, the incidence rates per 1,000 person-years among those who got the trivalent vaccine were 273.8, 294.1, and 373.4, respectively.

By comparison, the rates for those who received the live vaccine were 367.4, 381.5, and 450.1, respectively.

When the researchers adjusted for covariates, those rates yielded significant incidence rate ratios (in favor of the trivalent inactivated vaccine) of:

• 1.19 for the continuous cohort, with a 95% confidence interval from 1.17 to 1.22.
• 1.20 for the November cohort, with a 95% confidence interval from 1.17 to 1.23.
• And 1.10 for the intense cohort, with a 95% confidence interval from 1.07 to 1.13.

Although the study was not restricted to lab-confirmed flu, Dr. Eick said, the consistency of the results strengthens the study.

One possible explanation for the differences may be that military personnel are required to be vaccinated yearly, so that many may have had a pre-existing immunity that prevented the live vaccine from replicating, Dr. Eick said.

That explanation is biologically plausible, said session moderator John Treanor, M.D., of the University of Rochester Medical Center in Rochester, N.Y.

"Whether it turns out to be true remains to be seen," said Dr. Treanor, himself a renowned vaccine researcher. The theory has been that the live attenuated vaccine is effective in children because they have little or no pre-existing flu immunity.

Dr. Treanor added that the findings of the Army study are probably not strong enough to support decisions on which vaccine should be used in adults.

"The differences that people are seeing in adults, it seems to me, are probably not major enough to be clinically significant, although they may be biologically real," he said.