It appears likely that there will be a delay and possibly a reduction in the amount of influenza vaccine available for the 2000-2001 influenza season. One possible way of increasing the availability of influenza vaccine for this year is to use a half-dose in healthy adults. The objective of this study is to determine if the immune system responds to a half-dose the same way it responds to a whole dose. This study will use the currently approved inactivated influenza vaccine in healthy adults ages 18 to 49 years old.

If the immune response to the half-dose is not significantly different than the immune response generated to the whole dose, this could be a strategy to extend the amount of vaccine that could be available in this age group.

Eligible subjects will be randomized to receive a single intramuscular injection of either a half-dose or a whole dose of a currently approved influenza virus vaccine in their arm. Prior to vaccination, subjects will have one tube of blood drawn. Subjects will be given a 3-day diary card to record any symptoms they may experience and asked to return for a second and final visit approximately 21 days following vaccination. A second blood draw will be done.

Inclusion Criteria:

Volunteers may be eligible for this trial if they:

• Are able to and have given informed consent.
• Are able to understand and comply with all study procedures and to complete study diary.
• Are aged 18 to 49 years.
• Are female, and are not pregnant.
• Are available for all study visits.

Exclusion Criteria:

Volunteers will not be eligible for this trial if they:

• Are allergic to eggs or egg products.
• Have a current chronic medical condition for which influenza vaccine is normally recommended.
• Have received experimental vaccines or medications within 30 days of study entry.
• Have received parenteral immunoglobulin within 30 days of study entry.