Revaccination with smallpox possible with diluted vaccine.

2003 AUG 6 - (NewsRx.com & NewsRx.net) -- by Maria G. Essig, MS, ELS, senior medical writer - Adults can be revaccinated against smallpox relatively safely if the vaccine is diluted 10-fold, according to a report published in the Journal of the American Medical Association.

"There is renewed interest in use of smallpox vaccine due to the potential for a bioterrorist attack," said Sharon E. Frey and collaborators at St. Louis University and the EMMES Corporation in the United States. "This would involve vaccinating health care workers who were previously vaccinated."

Frey and her colleagues conducted a single-blind trial involving 80 adults, 32 to 60 years old, who had been previously vaccinated against smallpox. The researchers randomly assigned the subjects to receive the undiluted smallpox vaccine or a 1:3.2, 1:10, or 1:32 dilution of the vaccine. The results of revaccination were compared to those obtained from vaccination of 10 control subjects, aged 18 to 31 years, who had not received a previous vaccina inoculation.

"Initial vaccination resulted in a major reaction in 64 of 80 non-naive participants," reported the investigators.

The incidence of major reactions to revaccination was reduced by vaccine dilution. Among the subjects revaccinated with undiluted vaccine, 95% experienced a major reaction, compared to 90%, 81%, and 53% of the subjects who received the 1:3.2, 1:10, or 1:32 dilution, respectively. All 10 of the control subjects (100%) experienced major reactions to vaccination.

The revaccinated adults had significantly smaller skin lesions than did the newly vaccinated subjects (p=0.04) as well as a lower incidence of fever (p=0.02).

Most (95%) of the previously vaccinated subjects had circulating antibodies ...