Chiron Announces Promising Data from Clinical Study of Adjuvanted Avian Influenza Vaccine; Results Confirm Previous Clinical Studies: Chiron's MF59 Adjuvant Significantly Enhances Immune Response

The 96-patient study explored the safety and immunogenicity offour different doses of the investigational vaccine with and withoutChiron's adjuvant MF59. An adjuvant is a substance that is added to avaccine to boost the body's immune response to the vaccine's antigen.All vaccine formulations containing the adjuvant MF59 proved highlyimmunogenic, inducing antibody levels believed to confer protectionagainst the influenza strain. The lowest dose contained 3.75micrograms of antigen per dose, a quarter of the dose used in seasonalinfluenza vaccines. In marked contrast, the unadjuvanted vaccineinduced significantly lower antibody titers and did not reach levelsachieved by the adjuvanted vaccine following any of the antigen dosestested, which ranged from 3.75 to 30 micrograms.

"This study builds on the clinical data Chiron has alreadyestablished with adjuvanted influenza vaccines," said Rino Rappuoli,Ph.D., chief scientific officer of Chiron and head of research forChiron Vaccines. "With more than 20 million doses of MF59-adjuvantedvaccine distributed outside of the United States for seasonalvaccination, we have built an impressive safety database. In addition,our previous clinical studies of a vaccine designed to protect againstH5 avian influenza have demonstrated the potential of our adjuvant toreduce the required dose of antigen and to cross-protect against arange of related viral strains.

"This important collaborative research further advances ourunderstanding of the role innovative adjuvant technology can play inenhancing the immune response to pandemic vaccines, providing thepotential for dose-sparing to stretch production capacity," continuedDr. Rappuoli. "Following these successful results, we look forward toclinical studies of an adjuvanted H5N1 vaccine planned for the nearfuture."

"Adjuvants could be an important means of improving the immuneresponse to potential pandemic strains and allow us to stretchproduction capacity," added Walter A. Orenstein, M.D., professor ofmedicine of pediatrics and associate director of the Emory VaccineCenter in Atlanta, Georgia.

"Public-private partnerships are an essential part of ourpreparations for a potential pandemic, and this study clearlydemonstrates their value," said Dan Soland, president of ChironVaccines. "NIAID has taken the lead in sponsoring clinical trials todetermine the optimal characteristics of an effective and safepandemic vaccine. This trial builds on that knowledge, providingvaluable data on the importance of using adjuvants, which may becritical for extending the vaccine supply to reach as many people aspossible."

About Pandemic Influenza

Pandemic influenza occurs when a new influenza virus emerges thatis easily transmitted among humans and causes serious illness. In thissituation, the virus can result in a worldwide outbreak of disease, orpandemic. Pandemic influenza occurred three times in the last century.The 1918 pandemic killed at least 40 million people worldwide, with amortality rate of approximately 2.5 percent in the United States.

Avian influenza, or "bird flu," does not normally infect humans,but there have been several examples in recent years of transmissionto people, leading to fears of a strain with the potential to resultin a pandemic. The current outbreak of H5N1 avian influenza inSoutheast Asia has resulted in more than 100 human cases, in Cambodia,Indonesia, Thailand and Vietnam, with a mortality rate ofapproximately 50 percent. Despite the death or destruction of anestimated 150 million birds, the virus is now considered endemic in anumber of regions in Southeast Asia.

Media Materials

For images and video related to Chiron Vaccines, please visitwww.thenewsmarket.com/chiron. Journalists may register and downloadprint-quality images and broadcast-standard video from this site at nocharge.

About Chiron

Chiron delivers innovative and valuable products to protect humanhealth by advancing pioneering science across the landscape ofbiotechnology. The company works to deliver on the limitless promiseof science and make a positive difference in people's lives. For moreinformation about Chiron, please visit www.chiron.com.

This news release contains forward-looking statements, includingstatements regarding development of pandemic influenza vaccines andadjuvants, regulatory approvals, improvements to manufacturingfacilities, product development initiatives, and product marketing,which involve risks and uncertainties and are subject to change. Adiscussion of the company's operations and financial condition,including factors that may affect its business and future prospectsthat could cause actual results and developments to differ materiallyfrom those expressed or implied by these forward-looking statements,is contained in documents the company has filed with the SEC,including the Form 10-K for the year ended December 31, 2004, and theForm 10-Q for the quarter ended June 30, 2005, and will be containedin all subsequent periodic filings made with the SEC. These documentsidentify important factors that could cause the company's actualperformance to differ from current expectations, including, amongothers, litigation and investigations relating to influenza vaccines,the outcome of clinical trials, regulatory review and approvals,manufacturing capabilities, intellectual property protections anddefenses, litigation, stock-price and interest-rate volatility,marketing effectiveness, and the severity of the 2005-2006 influenzaseason. In particular, there can be no assurance that additionalissues with respect to influenza vaccines or Chiron's manufacturinggenerally will not arise in the future, or that Chiron will be able toincrease sales of existing products, successfully develop and receiveapproval to market new products (including pandemic influenzavaccines), or achieve market acceptance for such new products. Thecompany may face additional competition in the influenza market in thefuture and challenges in distribution arrangements as a result ofvaccine developments. In addition, the company may engage in businessopportunities, the successful completion of which is subject tocertain risks, including approval by Novartis AG, regulatory approvalsand the integration of operations.

Chiron does not undertake an obligation to update theforward-looking information the company is giving today.