This prospective annual release study was designed to assess the safety of a monovalent influenza virus vaccine using a new strain recommended for the 2009-2010 influenza season not previously contained in the trivalent intranasal FluMist vaccine. Three hundred healthy adults received a single dose of vaccine or placebo and were followed for 180 days.

This prospective, randomized, double-blind, placebo-controlled release study enrolled 300 healthy adults 18 to 49 years of age. Eligible participants were randomly assigned in a 4:1 fashion to receive a single dose of monovalent vaccine or placebo by intranasal spray. This study was conducted at multiple sites in the United States. Randomization was stratified by site.

Each participant received one dose of investigational product on Day 1. The duration of study participation for each participant was the time from receipt of investigational product through 180 days after receipt of investigational product.

Inclusion Criteria:

• Male or female, 18 to 49 years of age (not yet reached their 50th birthday) at the time of investigational product administration
• Healthy by medical history and physical exam
• Written informed consent and any locally required authorization (eg, Health Insurance Portability and Accountability Act [HIPAA] in the United States of America [USA], European Union [EU] Data Privacy Directive in the EU) obtained from the subject/legal representative prior to performing any protocol-related procedures, including screening evaluations
• Females of childbearing potential, unless surgically sterile (ie, bilateral tubal ligation, bilateral oophorectomy, or complete hysterectomy), had sterile male partner, were premenarchal or at least 2 years postmenopausal, or practiced abstinence, must have used 2 effective methods of avoiding pregnancy (including oral, transdermal, or implanted contraceptives, intrauterine device, female condom with spermicide, diaphragm with spermicide, cervical cap, or use of a condom with spermicide by the sexual partner) for at least 30 days prior to dosing with investigational product, and must have agreed to continue using such precautions for at least 90 days after dosing with investigational product; cessation of birth control after this point was to be discussed with a responsible physician. The subject must also have had a negative serum or urine pregnancy test within 14 days prior to investigational product administration (if screening and administration of investigational product did not occur on the same day) and on the day of investigational product administration prior to randomization
• Males, unless surgically sterile, must have used 2 effective methods of birth control with a female partner and must have agreed to continue using such contraceptive precautions for at least 30 days after dosing with investigational product (from Day 1 through Day 31 of the study)
• Subject available by telephone
• Ability to understand and comply with the requirements of the protocol, as judged by the investigator
• Ability to complete follow-up period of 180 days after dosing as required by the protocol

Exclusion Criteria:

• History of hypersensitivity to any component of the vaccine, including egg or egg protein or serious, life-threatening, or severe reactions to previous influenza vaccinations
• History of hypersensitivity to gentamicin
• Any condition for which the inactivated influenza vaccine was indicated, including chronic disorders of the pulmonary or cardiovascular systems (eg, asthma), chronic metabolic diseases (eg, diabetes mellitus), renal dysfunction, or hemoglobinopathies that required regular medical follow-up or hospitalization during the preceding year
• Acute febrile (> 100.0°F oral or equivalent) and/or clinically significant respiratory illness (eg, cough or sore throat) within 14 days prior to randomization
• Any known immunosuppressive condition or immune deficiency disease, including human immunodeficiency virus [HIV] infection, or ongoing immunosuppressive therapy
• History of Guillain-Barré syndrome
• A household contact who was severely immunocompromised (eg, hematopoietic stem cell transplant recipient, during those periods in which the immunocompromised individual required care in a protective environment); subject should additionally have avoided close contact with severely immunocompromised individuals for at least 21 days after receipt of investigational product
• Receipt of any investigational agent within 30 days prior to randomization, or expected receipt through 30 days after the dose of investigational product (use of licensed agents for indications not listed in the package insert was permitted)
• Expected receipt of anti-pyretic or analgesic medication on a daily or every other day basis from randomization through 14 days after receipt of investigational product
• Administration of intranasal medications within 14 days prior to randomization, or expected receipt through 14 days after administration of investigational product
• Known or suspected mitochondrial encephalomyopathy
• Nursing mother
• Any condition (eg, chronic cough, allergic rhinitis) that, in the opinion of the investigator, would have interfered with evaluation of the investigational product or interpretation of subject safety or study results
• Employees of the clinical study site or any other individuals involved with the conduct of the study, or immediate family members of such individuals

From the time that written informed consent was obtained, AEs were collected through Day 15 after vaccination and SAEs were collected through Day 181 after vaccination.