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Executive Summary

MERCK's PEDVAXHIB LOW SEROCONVERSION RATES FOR SOME LOTS of the haemophilus b conjugate vaccine have prompted the company to conduct additional lot testing. Merck notified FDA in June that during postmarketing studies of the vaccine some lots did not produce in certain children the level of antibodies considered protective (.15 mcg/ml). Merck said there are 12 suspect production lots that were manufactured in 1990 and expired in 1991. From those lots, 46,000 doses were administered in two clinical trials, one of which involved Native American children. The remaining 54,000 doses went to other trials or on the market. Some of the children received one dose from the suspect lots, while others received two doses, Merck said. Of the children receiving two doses of the vaccine, 85% of them seroconverted to the expected antibody level, Merck noted. Merck pointed out that the children are being closely monitored and that no increased incidence of disease has been seen. Despite lower antibody titers, other immune responses seemed to be protecting the children, the company noted. The children who did not initially produce the expected antibody level seroconverted after getting boosters. FDA said it believes Merck is doing everything it can to determine the source and scope of the problem. The agency noted that Merck plans to do an additional 30 tests that it does not routinely conduct on vaccine lots. The agency hopes to begin receiving data from Merck in August on chemical and biological tests on lots of the vaccine that did not produce the expected level of antibodies and from lots that had acceptable immunogenicity, the Center for Biologics Evaluation and Research said. CBER also expects to receive clinical data on children who were given doses from the suspect vaccine lots. All 12 suspected lots of PedvaxHIB passed Merck's manufacturing and animal immunogenicity tests, the company said. The lots also passed FDA's lot release tests, the agency noted. Merck pointed out that there is no release test that is predictive of what antibody level a haemophilus vaccine will achieve. The company said it is working to develop a predictive test. Merck stated that it does not know what caused the vaccine lots to produce low antibody titers, but that it is examining all possibilities, including the raw materials used, the conjugation processes, the manufacturing and filling process. Both FDA and Merck said that the agency has been releasing subsequent vaccine lots.

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