Business Wire - March 27, 2003

The pre-clinical findings in monkeys provide an early indication that the vaccine candidates used together in a complementary way, known as a prime-boost regimen, may be able to stimulate higher-level immune responses against HIV-1 than either vaccine used separately. Researchers for Merck and Aventis Pasteur say the Phase I clinical trial, which will be conducted in the U.S., will provide preliminary data about whether the combination of the two vaccine candidates can produce similar encouraging results in humans. Phase I represents the earliest stage of human testing for a potential vaccine.

In studies conducted in rhesus macaque monkeys, a prime-boost regimen with Merck's replication-defective adenovirus type 5 vector (Ad5) vaccine candidate given first, and Aventis Pasteur's canarypox virus vector (ALVAC) vaccine candidate given afterward, stimulated levels of cellular immune responses against HIV-1 that were higher than levels seen using the Ad5 vector alone, according to Emilio A. Emini, Ph.D., senior vice president of Vaccine Research at Merck. Dr. Emini will report the results of the pre-clinical studies at the Keystone Conference on HIV Vaccine Development, an annual gathering of leading HIV vaccine researchers, in Banff, Canada on March 31.

Details of the Pre-clinical Findings

Scientists evaluated vaccine-elicited cellular immune responses in the monkeys using the two vaccine candidates, both of which expressed the HIV-1 gag protein. Monkeys were vaccinated with the Ad5 vector and then subsequently boosted with Ad5 vector again or, alternatively, with the ALVAC vector. Use of the two different vectors resulted in immune responses that were superior to those seen using just the Ad5 vector alone. Specifically, the superior results were obtained when the Ad5 vector was used as the priming (first dose) vaccine and the ALVAC vector was used as the booster vaccine.

The findings, according to Merck and Aventis Pasteur scientists, were compelling enough to merit further study. "The results from the pre-clinical immunogenicity data in monkeys are encouraging," Dr. Emini said. "It is our hope that this collaboration may help enhance development and access to new vaccines."

Michel DeWilde, Ph.D., executive vice president of Research and Development at Aventis Pasteur, the vaccines business of Aventis, joined Dr. Emini in characterizing the collaboration. "We are extremely encouraged by the pre-clinical data and are moving to the clinic quickly," Dr. DeWilde said. "This is an unprecedented pairing between two companies with large, established HIV vaccine research programs. We hope that this collaboration will contribute to a solution to this devastating epidemic."

About Merck

Merck, whose HIV vaccine research program has been ongoing for almost 20 years, first began testing its HIV vaccine candidates in humans in late 1999. The MRKAd5 HIV-1 gag vaccine candidate is based on a modified common cold virus that is used as a means of delivering an HIV gene called gag into the cells to produce a potent cellular immune response to HIV.

In addition to the work being conducted as part of the collaboration with Aventis Pasteur, Merck is continuing its ongoing, independent research to develop an HIV vaccine. Phase I trials are underway in about half of U.S. states and the District of Columbia. About 1,300 uninfected and HIV-infected volunteers are participating in the studies, which are taking place at about 70 clinical sites. Later this year, Merck plans to begin the first international trial of its leading HIV vaccine candidate in conjunction with another organization, the National Institute of Allergy and Infectious Diseases (NIAID)-sponsored HIV Vaccine Trials Network (HVTN).

Merck & Co., Inc. is a global research-driven pharmaceutical products and services company. Merck discovers, develops, manufactures and markets a broad range of innovative products to improve human and animal health, directly and through its joint ventures. Merck headquarters are in Whitehouse Station, N.J., USA. For more information, please visit www.merck.com.

About Aventis

Aventis Pasteur has had one of the largest AIDS vaccine research programs in the world for nearly 15 years, developing vaccines for both prophylactic and therapeutic use. Focusing primarily on ALVAC-HIV, a canarypox vector into which HIV genes are inserted, the company's HIV vaccines have been studied in more than 40 clinical trials on five continents in more than 2,000 people since 1992. ALVAC-HIV was used in the first clinical trial undertaken in Africa as well as in landmark trials in the Caribbean and South America. ALVAC-HIV will be tested as part of a phase III clinical trial scheduled to begin in Thailand in 2003.

Aventis Pasteur is the largest company in the world devoted entirely to vaccines. In 2002, Aventis Pasteur produced 1.4 billion doses of vaccine, making it possible to protect 500 million people across the globe, which is about 1.4 million per day. The company offers the broadest range of vaccines, providing protection against 20 bacterial and viral diseases. Based in Lyon, France, Aventis Pasteur is owned by Aventis SA. For more information, please visit www.aventispasteur.com.

Aventis is dedicated to treating and preventing disease by discovering and developing innovative prescription drugs and human vaccines. In 2002, Aventis generated sales of $17.6 billion, invested $3.1 billion in research and development and employed approximately 71,000 people in its core business. Aventis corporate headquarters are in Strasbourg, France. For more information, please visit www.aventis.com.

Merck Forward-Looking Statement

This press release contains "forward-looking statements" as that term is defined in the Private Securities Litigation Reform Act of 1995. These statements involve risks and uncertainties which may cause results to differ materially from those set forth in the statements. The forward-looking statements include statements regarding product development and product potential. No forward-looking statement can be guaranteed, and actual results may differ materially from those projected. Additional detailed information concerning a number of factors that could cause actual results to differ materially is available in Item 1 of Merck's Annual Report on Form 10-K for the year ended Dec. 31, 2002, in its periodic reports on Form 10-Q and in its reports on Form 8-K (if any). Copies of these forms are available on request to Merck's Office of Stockholder Services.

Aventis Forward-Looking Statement

Statements in this news release other than historical information are forward-looking statements subject to risks and uncertainties. Actual results could differ materially depending on such factors as the availability of resources, the timing and effects of regulatory actions, the strength of competition, the outcome of litigation and the effectiveness of patent protection. Additional information regarding risks and uncertainties is set forth in the current Annual Report on Form 20-F of Aventis on file with the Securities and Exchange Commission.

CONTACT: For Merck Janet Skidmore (press), 908/423-3046 To reach Janet during the Keystone Conference: Mobile: 732/221-0390 or Mark Stejbach (investors), 908/423-5185 or For Aventis Pasteur U.S. Press: Cooney/Waters Beth Waters, 212/886-2200 To reach Beth during the Keystone Conference: Mobile: 917/770-2144 or In France: Dr. Robert Sebbag, +33-6-08172183

SOURCE: Merck & Co., Inc.

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