For the injected vaccine, the most common side effect is soreness at the injection site. Other side effects may include mild fever, body aches, and fatigue for a few days after the inoculation. For the nasal spray vaccine, the most common side effects include runny nose or nasal congestion for all ages, sore throats in adults, and -- in children 2 to 6 years old -- fever.

As with any medical product, unexpected or rare serious adverse events may occur. The FDA is working closely with governmental and nongovernmental organizations to enhance the capacity for adverse event monitoring, information sharing and analysis during and after the 2009 H1N1 vaccination program. In the U.S. Department of Health and Human Services, these agencies include the Centers for Disease Control and Prevention.

Vaccines against three seasonal virus strains are already available and should be used (see information on the seasonal flu). However, they do not protect against the 2009 H1N1 virus (see information on H1N1 flu).