Public Health

Frozen smallpox vaccine still effective after many decades

Last Updated: 2004-09-07 16:00:11 -0400 (Reuters Health)

NEW YORK (Reuters Health) - Even when diluted, the Aventis Pasteur smallpox vaccine (APSV), which has been in frozen storage since the 1950s, yields a high rate of successful vaccination, according to US researchers. Thus, dilution of this vaccine could help expand current stockpiles.

Dr. Kathryn M. Edwards, from Vanderbilt University in Nashville, and colleagues note that shortly after the September 11th attacks, smallpox vaccination resumed in the US and the goal was to have enough doses to vaccinate everyone. However, stocks were inadequate.

The researchers observe in the September 8th issue of the Journal of the American Medical Association that it was recently shown that a lyophilized form of the vaccine could be diluted without significantly altering its efficacy. Although this helped expand the stockpile, there still was not enough vaccine. Dilution of the stored lots of APSV could help bridge the coverage gap, but it was unclear if the vaccine would retain its effectiveness.

To investigate, the team assessed the outcomes of 340 vaccinia-na�ve adults who were randomized to receive APSV at one of three strengths: undiluted, 1:5, and 1:10. Successful vaccination was defined as the presence of a vesicle or pustule at the injection site 6 to 11 days after vaccination.

Overall, 99.4% of subjects had a successful vaccination and the success rates did not differ significantly between the groups.

Local and systemic reactions occurred with similar frequency in the three groups. Nearly all of the subjects reported at least one symptom at the injection site. Axillary lymphadenopathy was noted in 61.8% of subjects, fever in 21.5%, satellite lesions in 15%, and distant rash in 7.6%. Twenty-five percent of subjects reported missing scheduled activities because of vaccine-related symptoms.

"Although manufactured nearly 50 years ago, APSV is associated with high vaccination rates, even at a 1:10 dilution," the authors conclude. "Therefore, the existing supply of approximately 85 million doses of APSV can be expanded, leaving an ample stockpile of smallpox vaccine to protect the entire US population in the event widespread vaccination is imminently needed."

JAMA 2004;292:1205-1212.