The Food and Drug Administration has approved 4 vaccines against the H1N1 flu virus (formerly known as swine flu) for 2009. The FDA expects the vaccines to be distributed nationally in October.

Based on preliminary data from adults participating in clinical studies, the 2009 H1N1 vaccines induce a robust immune response in most healthy adults 8 to 10 days after a single dose, as occurs with the seasonal influenza vaccine.   

As with the seasonal influenza vaccines, the 2009 H1N1 vaccines are being produced in formulations that contain thimerosal, a mercury-containing preservative, and in formulations that do not contain thimerosal.   The FDA said that people with severe or life-threatening allergies to chicken eggs, or to any other substance in the vaccine, shouldn't be vaccinated.

The Centers for Disease Control and Prevention (CDC) has recommended that H1N1 vaccination efforts focus on 5 key groups of people:

• Pregnant women,
• People who live with or care for children younger than 6 months of age,
• Healthcare and emergency medical services personnel,
• Individuals between the ages of 6 months through 24 years of age, and
• People from ages 25 through 64 years who are at higher risk for novel H1N1 because of chronic health disorders or compromised immune systems.

The agency said that the following groups should receive the vaccine before others if the vaccine is initially available in extremely limited quantities:

• Pregnant women,
• People who live with or care for children younger than 6 months of age,
• Healthcare and emergency medical services personnel with direct patient contact,
• Children 6 months through 4 years of age, and
• Children 5 through 18 years of age who have chronic medical conditions.

The CDC said that once the demand for vaccine for the prioritized groups has been met at the local level, programs and providers should begin vaccinating everyone from ages 25 through 64 years.

Health officials are recommending that the H1N1 vaccine be used alongside the seasonal flu vaccine.