New Data Presented at ICAAC Shows Aviron's FluMist(TM) Intranasal Influenza Vaccine Provided 86% Protection Against a Variant Strain Of Influenza in Children

A/Sydney Strain Prevalent in '97-'98 Season Not Included in Vaccine

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SAN DIEGO, Sept. 27, 1998 -- New data from the second year of a Phase 3 efficacy trial in children shows that Aviron's (Nasdaq: AVIR) FluMist(TM) intranasal influenza virus vaccine, under development for flu prevention, provided 100 percent protection against culture-confirmed influenza strains included in last year's flu vaccine, and 86 percent protection against the predominant strain of influenza circulating during last year's flu season, A/Sydney. This trial was conducted in collaboration with the National Institute of Allergy and Infectious Diseases (NIAID), a division of the National Institutes of Health.

The A/Sydney influenza strain was not included in the 1997-98 formulation because FluMist(TM) was designed to match the flu shot marketed that year. The flu shot was not studied in this trial.

The data were presented today at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) in San Diego by Robert Belshe, M.D., lead investigator and Director of Infectious Diseases and Immunology, and Director of the Center for Vaccine Development at Saint Louis University, St. Louis, MO. The company plans to submit these data to the U.S. Food and Drug Administration (FDA) for review.

"Live attenuated virus vaccines such as FluMist(TM) are thought to mimic natural infection better than other types of vaccines. In addition, FluMist(TM) is delivered to the upper airway, the natural port of entry for the influenza virus," Dr. Belshe said.

Results from the double-blind placebo-controlled study show that overall, FluMist(TM) provided 87 percent protection against all culture-confirmed influenza. In the 1358 participants, there were five cases of influenza due to influenza strains included in the vaccine and 66 cases caused by A/Sydney. Only two percent of children vaccinated with FluMist(TM) (15 out of 917) experienced culture-confirmed influenza, all of which was attributable to the A/Sydney strain, while 13 percent of the placebo recipients (56 out of 441) experienced culture-confirmed influenza. The difference between these two influenza attack rates is used to calculate the overall protection rate of 87 percent.

The incidence of pneumonia and other lower respiratory diseases was also reduced, compared to placebo. Eight children in the placebo group developed influenza-related wheezing, bronchitis or pneumonia. All of these were due to the A/Sydney strain. No children who received FluMist(TM) experienced lower respiratory complications.

These new data are from the second year of a randomized, placebo-controlled study in healthy children initiated during the 1996-97 flu season. The children were invited back to participate for a second flu season in the 1997-98 follow-up trial. They were either vaccinated with a single dose of FluMist(TM) or a placebo spray. Of the 15 children out of 917 children in the FluMist(TM) group who did contract influenza, the illness appeared to be milder than in the control group, based on frequency of complications and duration of fever.

Cross-Protection Against the A/Sydney Strain
"Considering the unexpected emergence and severity of the A/Sydney strain during last year's flu season, we believe the observed level of protection is especially important," said J. Leighton Read, M.D., chairman and chief executive officer of Aviron. "It confirms the results we have seen in the past and indicates that FluMist(TM) may provide protection against strains of influenza somewhat different from those in the annual formulation."

Each year, the Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO), and the FDA determine which three strains of the influenza virus are likely to infect the population in the coming flu season. The vaccine formulation is then updated to include the most current influenza virus strains. Reports published in the CDC's MMWR (Morbidity and Mortality Weekly Report) indicate that there were numerous outbreaks last year of A/Sydney influenza, a variant of one of the strains included in the vaccine. A/Sydney was not included in the 1997-98 vaccine, and these reports indicate the protection provided against this strain by the vaccine may have been low.

Reduction of Otitis Media
The new data also indicate that FluMist(TM) provided 94 percent protection against influenza-related otitis media (two cases in the vaccine group vs. 17 in the placebo group). Otitis media -- a middle ear infection that is often linked to respiratory illnesses -- is the most common illness in young children requiring a doctor visit.

Results from the year-one study (1996-97 flu season) published in the May 14, 1998 New England Journal of Medicine, showed that only one percent (14 of 1070) of the children who received FluMist(TM) developed culture-confirmed influenza, versus 18 percent (95 of 532) of the children who received placebo -- a protection rate of 93 percent. The first year data also indicated that the vaccine provided 98 percent protection against influenza-associated otitis media.

FDA Submission Status
The data presented today will be included when Aviron resubmits its application for FluMist(TM) to the FDA. Aviron's Product License Application/Establishment License Application for FluMist(TM) was originally submitted to the FDA on June 30, 1998. On August 31, Aviron reported that it received notice from the FDA that its application was not accepted for filing due to lack of data on manufacturing, validation and stability. These developments make it unlikely that FluMist(TM) will be available for the 1999/2000 flu season. Aviron intends to seek U.S. licensure for FluMist(TM) to prevent influenza and its complications in children and adults.

The Impact of Influenza
Thirty-five to 50 million Americans become infected with the virus each flu season, which typically lasts from November to March. Annually, an estimated 20,000 Americans die from influenza and its complications. In addition to the serious health risks associated with influenza, the illness disrupts lives, productivity and has a significant economic impact. Each year, between 20-66 million work days are lost and 15-35 million school days are missed as a result of the influenza virus. An estimated $4.6 billion is spent annually on direct medical costs related to influenza in the United States.

Additionally, each year, otitis media is responsible for more than 31 million visits to doctors (approximately one-third of all pediatric office visits) and $3-4 billion in treatment costs. In addition to medical costs, work hours lost by parents caring for sick children contribute to the high cost of otitis media.

This trial was conducted under a Collaborative Research and Development Agreement (CRADA) between Aviron and the National Institute of Allergy and Infectious Diseases (NIAID). It involved 10 sites nationwide. The principal investigators included six Vaccine and Treatment Evaluation Units (VTEUs) supported by NIAID and four Aviron supported sites: Robert Belshe, M.D., Center for Vaccine Development at Saint Louis University School of Medicine, St. Louis, MO; David Bernstein, M.D., Children's Hospital Medical Center, Cincinnati, OH; Stan L. Block, M.D., Kentucky Pediatric Research Institute, Inc., Bardstown, KY; William C. Gruber, M.D., Vanderbilt University, Nashville, TN; Frederick Hayden, M.D., University of Virginia Health Sciences Center, Charlottesville, VA; James King, M.D./Karen Kotloff, M.D., University of Maryland at Baltimore, Baltimore, MD; Pedro Piedra, M.D., Influenza Research Center, Baylor University College of Medicine, Houston, TX; Keith Reisinger, M.D., Pittsburgh Pediatric Research, Pittsburgh, PA; John Treanor, M.D., Vaccine Evaluation Unit at the School of Medicine and Dentistry at University of Rochester, Rochester, NY; and Ken Zangwill, M.D., Harbor-UCLA Research and Education Institute, Torrance, CA.

The National Institute of Allergy and Infectious Diseases, a division of the National Institutes of Health, conducts and supports research to prevent, diagnose, and treat infectious diseases. NIAID has a major responsibility within the federal government for developing vaccines to control and prevent infectious diseases. NIH is an agency of the U.S. Department of Health and Human Services.

Aviron is a biopharmaceutical company based in Mountain View, CA focused on prevention of disease. The company's goal is to develop products which offer cost-effective prevention of a wide range of infections that affect the general population. The majority of Aviron's products under development are live vaccines against viral infections. These include intranasal vaccines for respiratory infections and their complications -- influenza, parainfluenza (PIV-3), and respiratory syncytial virus (RSV), and injectable vaccines to prevent cytomegalovirus (CMV) and genital herpes (HSV-2). Aviron is also developing, in collaboration with SmithKline Beecham Biologicals, a subunit vaccine against Epstein-Barr Virus (EBV) infection, a major cause of infectious mononucleosis.

This press release contains forward-looking statements. Actual results may differ materially from those suggested here. Additional information concerning factors that could cause such a difference is contained in Aviron's Annual Report on Form 10-K for the year ended December 31, 1997.

To receive an index and copies of recent press releases, call Aviron's News-On-Call toll-free fax service, 800-758-5804, extension 114000.

CONTACT: Additional information about the company can be located at http://www.aviron.com.