less reactogenic vaccine, dose response studies suggest that increasing the hemagglutinin content to a much higher level would produce only a small increase in antibody response.

The committee considers the probability that either a more purified hemagglutinin and neuraminidase vaccine or a live attenuated cold-adapted vaccine could be developed successfully to be very high, with a 90 percent probability of success within three to four years. If the vaccines contain the relevant antigens of the (natural) challenge viruses, their efficacy probably will be about 85 percent. Inactivated vaccine will have to be given annually. Until more data have been collected to demonstrate that a live attenuated vaccine protects against drift for periods longer than one year, annual administration probably will be required for this type of vaccine also.

The National Institute of Allergy and infectious Diseases (NIAID) has supported the development of the cold-adapted live attenuated influenza virus up to the point of large scale clinical trials. No similar attempt has been made to develop a more purified hemagglutinin and neuraminidase inactivated preparation. The cost of clinical trials for influenza vaccines is large. Efficacy studies are complicated by the unpredictability of the challenge (circulating) virus. A population might be immunized with an experimental vaccine in a year in which no natural influenza outbreak occurs. This is a generic problem for all natural challenge studies in vaccine recipients, but is more problematic with influenza vaccines than with others.

Another practical problem associated with the live attenuated vaccine is the intranasal route of administration. Clinical researchers also may have difficulty demonstrating that the vaccine is safe and effective for high-risk patients, e.g., those with severe lung disease. One of the primary concerns expressed about current vaccines is the relative lack of data demonstrating their consistent efficacy in high-risk populations. Most efficacy studies have been performed in healthy young adults. If future vaccines are shown to be more effective for high-risk groups than current vaccines, they probably will be accepted more widely by both providers and the lay public.

As summarized above, the major effort to develop new influenza vaccines has been at the NIAID. It appears that the pharmaceutical industry does not anticipate widespread use and acceptability of a live attenuated vaccine in the near future. Members of the committee are not aware of any major commercial efforts in this field.

Predictions on the further development of vaccines for influenza A and B appear in Chapter 5.

The health belief model parameters (perceptions of risk of illness, severity, vaccination benefits, and barriers) used to predict vaccine utilization are described in Chapter 6, in which scores assigned to various vaccines are displayed together for comparison.