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November 2, 2009

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Updated Results: In Youngest Children, a Second Dose of
2009 H1N1 Influenza Vaccine Elicits
Robust Immune Response

The National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, today announced interim results that show that children nine years of age and younger have a significantly improved immune response when given a second 15-microgram dose of 2009 H1N1 influenza vaccine.

The clinical trial evaluated the immune response of children six months to 17 years of age who received two doses of either 15 or 30 micrograms of 2009 H1N1 influenza vaccine. One of the most important findings from this study is that among children nine years old and younger, the second dose elicited a robust immune response after eight to 10 days, a significant improvement over the immune responses in this age group following only a single dose.

These findings support the current recommendation of the Advisory Committee on Immunization Practices (ACIP), which sets U.S. recommendations for all immunizations: To achieve an immune response likely to protect from illness, children nine years of age and younger should receive two 15-microgram doses of 2009 H1N1 vaccine. The trial data also continue to support the recommendation that children ten years of age and older should receive only one 15-microgram dose of vaccine.

The interim results include data from all available samples from the 583 children enrolled into the trial, and build upon the preliminary results released September 21 from a subset of volunteers. Those early results, based on blood samples taken eight to 10 days following the first injection, showed that in the majority of children 10 years of age and older, a single 15-microgram dose of vaccine elicited a strong immune response. In contrast, most of the younger children did not respond strongly to one dose of either 15 or 30 micrograms of vaccine.

The new data, obtained eight to 10 days after the second vaccine dose, are compared here with results obtained 21 days after the first vaccine dose:

  • Among the youngest children (6 to 35 months), 100 percent had a robust immune response after the second 15-microgram dose compared with only 25 percent three weeks after the first dose.
  • In children aged 3 through 9 years old, 94 percent had a robust response after the second 15-microgram dose compared with only 55 percent three weeks after the first dose.
  • In general, the immune responses in children receiving two 15-microgram doses and those receiving two 30-microgram doses of vaccine were similar, suggesting that receipt of two 15 microgram doses is adequate to elicit a strong immune response.

Analysis continues on the data from this trial, and additional findings will be provided as they become available.

Additional information about this and other NIAID-sponsored trials of 2009 H1N1 influenza vaccines are available at the NIAID's H1N1 Flu page.

Media inquiries can be directed to the NIAID Office of Communications at 301-402-1663,

NIAID conducts and supports research—at NIH, throughout the United States, and worldwide—to study the causes of infectious and immune-mediated diseases, and to develop better means of preventing, diagnosing and treating these illnesses. News releases, fact sheets and other NIAID-related materials are available on the NIAID Web site at

About the National Institutes of Health (NIH): NIH, the nation's medical research agency, includes 27 Institutes and Centers and is a component of the U.S. Department of Health and Human Services. NIH is the primary federal agency conducting and supporting basic, clinical, and translational medical research, and is investigating the causes, treatments, and cures for both common and rare diseases. For more information about NIH and its programs, visit

NIH...Turning Discovery Into Health ®

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Last Updated November 02, 2009