The objective of this study is to describe the safety and immunogenicity of a non-adjuvanted vaccine against pandemic influenza A (H1N1)in patients with chronic and or immunocompromised disease, elderly and pregnants. The primary immunological endpoint is to analyze the proportion of subjects with antibody titers of 1:40 or more on hemagglutination-inhibition assay 21 days after 1 dose of the vaccine. Volunteers will be monitored for safety during 21 days after vaccination. Volunteers will be recruited based on inclusion and exclusion criteria. Vaccine composition is: 15 micrograms of split inactivated virus (A/California/7/2009 (H1N1) (NYMC X179A). The hypothesis of the study is that the vaccine is safe and immunogenic in the volunteers recruited.

Inclusion Criteria:

• Volunteers able to understand and agree to participate in the study.

Exclusion Criteria:

• Have egg allergy
• Have past history of allergy to seasonal influenza vaccine
• Have received another inactivated vaccine within the prior 2 weeks or live vaccine in the past four weeks to his/her participation in the study
• Acute infectious disease during seven days prior vaccination
• Confirmed prior infection by pandemic influenza A (H1N1)
• Have participated in another clinical trial in the last 6 months
• Any other condition identified by the principal investigators which is considered not safe for enrollment of the volunteer.

Volunteers with chronic and or immunocompromised disease, elderly and pregnant in follow - up at the institutions participants in the study will be recruited.