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SWISS SERUM & VACCINE INSTITUTE's TY21A THYPHOID CAPSULE VACCINE

Executive Summary

SWISS SERUM & VACCINE INSTITUTE's TY21A THYPHOID CAPSULE VACCINE: licensure should not be delayed for results of ongoing trials with a new liquid formulation of the vaccine, FDA's Vaccine & Related Biological Products Advisory Committee agreed at its Oct. 30 meeting. The Swiss firm currently has a license application under review at FDA for an enteric coated capsule typhoid vaccine, but has just begun two trials which will compare the enteric coated capsule product to a potentially more efficacious liquid formulation of the vaccine. The capsule vaccine "apparently [has] greater safety and perhaps acceptability" than the parenteral typhoid vaccine now marketed in the U.S., FDA Division of Bacterial Products Staffer William Habig, PhD, told the committee. Wyeth markets the parenteral vaccine. Habig asked the committee if "there is any justification for waiting an additional two or three years to get the results of a comparative study of [oral] fluid versus enteric formulation when in fact, at that time, we might find the enteric product is just as good anyway?" Responding, Committee Chairman Kenneth McIntosh, MD, Children's Hospital Medical Center, Boston, said that licensure of the capsule vaccine should not be held up, "but we're very pleased that they are looking at other forms of the vaccine." Swiss Serum's enteric coated capsule vaccine is currently approved for use in approximately 30 countries. The firm reported that of 15 mil. doses distributed since 1983, there have been 11 adverse reactions reported as of July, 1986. The high safety profile of the vaccine was also discussed at the advisory committee's May 1985 meeting ("The Pink Sheet, June 3, 1985, p. 15). Both of the liquid formulation trials began in October, 1986. In one trial, schoolage children in Santiago, Chile were randomized to receive the capsule vaccine, a capsule placebo, the liquid vaccine, or a liquid placebo. Approximately 59,000 children are in each of the vaccine-treated groups, while 8,500 children are in each of the placebo groups. In the second trial, four groups of 6,000 people (aged 6 to 45) in Indonesia were randomized to receive the vaccines or placebos. Efficacy results for the two trials will be evaluated after three years. The Swiss Serum & Vaccine Institute, under grants from the World Health Organization and other health groups, has conducted a number of field trials with its typhoid vaccine. One of the first trials, conducted in Alexandria, Egypt, found a 96% efficacy rate over a three-year period for a liquid formulation of the TY21a vaccine. Subsequent trials conducted in Chile, using the enteric coated capsules, yielded efficacy rates of approximately 70%. The firm said that the efficacy differences could be related to different attack rates in the two countries. Chile has a higher rate of typhoid than Egypt. Stanley Cryz, PhD, who heads Swiss Serum & Vaccine Institute's bacteriology department, explained that the new comparative trials were designed to evaluate whether the efficacy differences seen in the earlier trials were related to the different formulations used or to the different attack rates in the two countries. Cryz said a liquid formulation could, theoretically, provide better protection from typhoid than a capsule product because of the absorption rate of a liquid compared to a capsule. "We don't know in vitro or in vivo how rapidly the enteric coated capsules open up in the small intestine -- whether it's a massive release of the bacteria [or] the bacteria leach out, and in theory that could have a profound effect on the degree of immunity," Cryz stated.

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