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MONDAY, Aug. 18, 2003 (HealthDayNews) -- A newer and hopefully safer smallpox vaccine is about to undergo the final test that would make it available for widespread use, researchers at a Massachusetts biotechnology company report.
The vaccine, which uses a clone of the smallpox virus designated ACAM 1000, could lessen the risk of encephalitis, a major side effect of the existing vaccine, says Dr. Thomas P. Monath, chief scientific officer of Acambis, Inc.
Acambis has already produced 150 million doses of the new vaccine, which are being stored by the federal government, Monath says. It has undergone the preliminary tests required for approval by the U.S. Food and Drug Administration, and plans are being made for the "pivotal phase III trial," he says. Phase III is the final stage of clinical trials that test both the safety and efficacy of medicines or devices.
A report describing the evolution of the new vaccine is being published in the September issue of the journal Nature Medicine, just five days after an expert panel of the federal Institute of Medicine said the risks of vaccinating millions of Americans with the existing vaccine would outweigh the benefits.
Monath is careful to say that the ACAM 1000 vaccine would not eliminate all those risks. The belief that it would have a lesser risk of encephalitis is based on animal tests and must be verified in the phase III trial, in which its safety and effectiveness would be tested against the existing Dryvax vaccine in 5,000 people, he says.
But there are some clear advantages of the newer vaccine, Monath says. Virus for the Dryvax vaccine was grown in the skin of calves, which now is unacceptable. The ACAM 1000 vaccine uses virus grown in tissue culture.
And while Dryvax used a melange of smallpox vaccine strains, which means that "one can outgrow the others and cause side effects," ACAM 1000 uses a single purified strain, selected for maximum protection and minimum side effects, Monath says.
Smallpox vaccination has been in the headlines since the Bush administration announced plans to immunize 500,000 military personnel, and a voluntary program to vaccinate several million medical and emergency workers. That program is lagging badly, with only 38,000 people volunteering to get the shot.
Acambis is not making sweeping claims about the ACAM 1000 vaccine. The journal report says cautiously that it "probably represents an advance over Dryvax in terms of purity, quality, freedom from bacterial contamination allowed in the old vaccine ... and a possible lower incidence of postvaccinal encephalitis."
But a much safer vaccine could be on the way, Monath says. It was developed in Germany in the 1970s, "at the end of the smallpox era," he says, and uses a live virus that cannot multiply. It would be reserved for persons with medical conditions that would preclude the use of the standard vaccine.
Another study, appearing in the same issue of the journal, finds that the older vaccine provides protection for much longer than has been thought. Detailed studies of 18 people who were vaccinated before the program ended in 1978 shows that "the majority still have some degree of protection," says the study's leader, Dr. Mark K. Slifka, an assistant professor in the Oregon Health and Sciences University Vaccine Therapy Institute.
Slifka and his colleagues looked at the two aspects of immune system protection, cellular and antibody. They report that antibodies, molecules that would respond to a smallpox attack, appear to stay active for decades, perhaps as long as 75 years.
Cellular protection is not as long-lasting, they report, but it declines slowly, with a half-life between eight and 15 years. And so any terrorist attack with smallpox would not be as devastating as feared because of "pre-existing immunity in a large number of previously vaccinated individuals," they say.
More information
Read more about smallpox from the Centers for Disease Control and Prevention or the Center for Civilian Biodefense Strategies at Johns Hopkins University.
