Stored smallpox vaccine effective after dilution

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, new research shows. Therefore, dilution of this vaccine could help expand current stockpiles.

Shortly after the September 11th attacks, smallpox vaccination resumed in the US with the goal of having enough doses so that every citizen could be vaccinated in the event of a biological attack. However, the stockpile was inadequate to meet this goal.

Recently, it was shown that a form of the vaccine introduced in the 1970s could be diluted without significantly weakening its effects. Although this helped expand the stockpile, there still was not enough vaccine for every citizen. Dilution of the stored lots of APSV, which were first reported by the drugmaker in 2001, could help bridge the coverage gap, but it was unclear it the vaccine would retain its effectiveness.

To investigate, Dr. Kathryn M. Edwards, from Vanderbilt University in Nashville, and colleagues assessed the outcomes of 340 adults who were given APSV at one of three strengths: undiluted, 1-to-5 dilution, and 1-to-10 dilution.

Overall, 99 percent of subjects had a successful vaccination and the success rates did not differ significantly between the groups, the investigators report in the Journal of the American Medical Association for September 8.

Skin reactions occurred with similar frequency in the three groups. Nearly all of the subjects reported at least one symptom at the injection site.

"Although manufactured nearly 50 years ago, APSV is associated with high vaccination rates, even at a 1:10 dilution," the authors state. "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."

SOURCE: Journal of the American Medical Association, September 8, 2004.