NEW YORK (Reuters Health) – The results of a new study suggest that just one dose of the vaccinia vaccine against smallpox provides immunity for as long as 88 years -- which has implications if smallpox were used as a biological weapon.

In the event of a biological attack with smallpox, the current findings suggest that smallpox vaccine should first be given to individuals who have never been immunized before, Dr. Dan L. Longo and colleagues, from the National Institutes of Health, Bethesda, Maryland, advise.

As reported in The American Journal of Medicine for December, the researchers assessed antiviral antibody immunity from smallpox vaccination in 209 participants of the Baltimore Longitudinal Study of Aging (BLSA) who had been vaccinated 13 to 88 years previously.

In these individuals, high vaccinia-specific IgG and neutralizing antibodies levels persisted essentially forever, the results indicate. Antivaccinia IgG titers ranged from 1:32 to 1:256 and neutralizing antibody titers were similar in the majority of individuals. The absolute level of antivaccinia antibodies was only slightly higher when multiple doses of vaccine rather than one dose had been given.

A separate analysis of BLSA participants who had not been vaccinated, but had survived a smallpox infection in their youth, revealed that their antivaccinia antibody titers were comparable to those seen in vaccinated subjects.

"These data imply that limited supplies of vaccine can be more usefully applied (perhaps in diluted form) to individuals who have never been vaccinated, primarily individuals born after 1972," the authors state.

Am J Med 2008;121:1058-1064.