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Published at www.nejm.org September 10, 2009 (10.1056/NEJMoa0906453)

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ABSTRACT

Background A new pandemic influenza A (H1N1) virus has emerged, causing illness globally, primarily in younger age groups. To assess the level of preexisting immunity in humans and to evaluate seasonal vaccine strategies, we measured the antibody response to the pandemic virus resulting from previous influenza infection or vaccination in different age groups.

Methods Using a microneutralization assay, we measured cross-reactive antibodies to pandemic H1N1 virus (2009 H1N1) in stored serum samples from persons who either donated blood or were vaccinated with recent seasonal or 1976 swine influenza vaccines.

Results A total of 4 of 107 persons (4%) who were born after 1980 had preexisting cross-reactive antibody titers of 40 or more against 2009 H1N1, whereas 39 of 115 persons (34%) born before 1950 had titers of 80 or more. Vaccination with seasonal trivalent inactivated influenza vaccines resulted in an increase in the level of cross-reactive antibody to 2009 H1N1 by a factor of four or more in none of 55 children between the ages of 6 months and 9 years, in 12 to 22% of 231 adults between the ages of 18 and 64 years, and in 5% or less of 113 adults 60 years of age or older. Seasonal vaccines that were formulated with adjuvant did not further enhance cross-reactive antibody responses. Vaccination with the A/New Jersey/1976 swine influenza vaccine substantially boosted cross-reactive antibodies to 2009 H1N1 in adults.

Conclusions Vaccination with recent seasonal nonadjuvanted or adjuvanted influenza vaccines induced little or no cross-reactive antibody response to 2009 H1N1 in any age group. Persons under the age of 30 years had little evidence of cross-reactive antibodies to the pandemic virus. However, a proportion of older adults had preexisting cross-reactive antibodies.

Source Information

From the Influenza Division, National Center for Immunization and Respiratory Diseases (K.H., V.V., X.L., W.Z., E.N.B., H.S., F.L., L.D., P.M.G., T.L.B., N.J.C., T.M.T., J.M.K.), and the Division of Global AIDS, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention (J.R.D.) — all at the Centers for Disease Control and Prevention (CDC), Atlanta.

This article (10.1056/NEJMoa0906453) was published on September 10, 2009, at NEJM.org.

Address reprint requests to Dr. Katz at the Influenza Division, CDC, 1600 Clifton Rd., Atlanta, GA 30333, or at jkatz{at}cdc.gov.

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