Business Wire - September 17, 2003

The collaborative Phase I study looks at a prime-boost regimen that in an earlier preclinical investigation in monkeys found a higher level of immune response against HIV-1 than either vaccine used separately. Phase I represents the earliest stage of human testing for a potential vaccine.

The prime-boost regimen uses Merck's replication-defective adenovirus type 5 vector (Ad5) vaccine candidate first, followed by the canarypox virus vector (ALVAC) vaccine candidate from Aventis Pasteur, the vaccines business of Aventis. Data presented earlier this year at the 2003 Keystone Conference on HIV Vaccine Development demonstrated that the combination stimulated levels of cellular immune responses against HIV-1 that were higher than levels seen using the Ad5 vaccine candidate alone in a study of monkeys. The Phase I study will determine if a similar synergistic immune response can be generated in humans.

The study will include 17 sites across the United States. Volunteers participating in the study have received Merck's Ad5 vaccine candidate in previous clinical trials. In this study, they will receive a boost regimen of Aventis Pasteur's ALVAC vaccine candidate. After receiving the boost vaccine candidate, volunteers will have blood drawn and analyzed to determine the level of cellular immune responses generated by the candidate vaccines.

In the preclinical studies, rhesus macaque monkeys were vaccinated with the Ad5 vaccine candidate and subsequently boosted with the Ad5 vaccine candidate again or, alternatively, with the ALVAC vaccine candidate. Use of the two different vaccine candidates resulted in immune responses that were superior to those seen using just the Ad5 vaccine candidate alone. Specifically, the superior results were obtained when the Ad5 vaccine candidate was used as the priming (first dose) vaccine and the ALVAC vaccine candidate was used as the booster (second dose) vaccine.

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. Merck's replication-defective adenovirus type 5 vector (Ad5) 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 800 uninfected and HIV-infected volunteers are participating in the studies, which are taking place at about 70 clinical sites. This year, Merck began the first international trial of its Ad5 vaccine candidate in conjunction with another organization, the National Institute of Allergy and Infectious Diseases-sponsored HIV Vaccine Trials Network (HVTN). That trial, which involves both HVTN and Merck sites, is taking place in five regions of the world, including 18 cities in seven countries affected by HIV.

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.

About Aventis

Aventis is dedicated to treating and preventing disease by discovering and developing innovative prescription drugs and human vaccines. In 2002, Aventis generated sales of EUR 17.6 billion, invested EUR 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.

The vaccines business of 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.

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. For more information, please visit www.aventispasteur.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 containing projections or estimates of revenues, income, earnings per share, capital expenditures, capital structure, or other financial items; plans and objectives relating to future operations, products, or services; future economic performance; or assumptions underlying or relating to any such statements, are forward-looking statements subject to risks and uncertainties. Actual results could differ materially depending on factors such as the timing and effects of regulatory actions, the results of clinical trials, the company's relative success developing and gaining market acceptance for new products, the outcome of significant 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 and in the current Annual Report -- "Document de Reference" -- on file with the "Commission des Operations de Bourse" in France.

CONTACT: Merck & Co., Inc.

Press:

Janet Skidmore, 908-423-3046 or 732-221-0390

or

Investor Relations:

Mark Stejbach, 908-423-5185

or

Aventis Pasteur

U.S. Press:

Cooney/Waters

Beth Waters, 212-886-2200 or 917-770-2144

or

In France:

Dr. Robert Sebbag, 011-33-6-0817-2183

or

Investor Relations:

Dr. Philippe Zeisser, 011-33-38899-1171

SOURCE: Merck & Co., Inc.

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