Background A new pandemic influenza A (H1N1) virus has emerged,causing illness globally, primarily in younger age groups. Toassess the level of preexisting immunity in humans and to evaluateseasonal vaccine strategies, we measured the antibody responseto the pandemic virus resulting from previous influenza infectionor vaccination in different age groups.
Methods Using a microneutralization assay, we measured cross-reactiveantibodies to pandemic H1N1 virus (2009 H1N1) in stored serumsamples from persons who either donated blood or were vaccinatedwith recent seasonal or 1976 swine influenza vaccines.
Results A total of 4 of 107 persons (4%) who were born after1980 had preexisting cross-reactive antibody titers of 40 ormore against 2009 H1N1, whereas 39 of 115 persons (34%) bornbefore 1950 had titers of 80 or more. Vaccination with seasonaltrivalent inactivated influenza vaccines resulted in an increasein the level of cross-reactive antibody to 2009 H1N1 by a factorof four or more in none of 55 children between the ages of 6months and 9 years, in 12 to 22% of 231 adults between the agesof 18 and 64 years, and in 5% or less of 113 adults 60 yearsof age or older. Seasonal vaccines that were formulated withadjuvant did not further enhance cross-reactive antibody responses.Vaccination with the A/New Jersey/1976 swine influenza vaccinesubstantially boosted cross-reactive antibodies to 2009 H1N1in adults.
Conclusions Vaccination with recent seasonal nonadjuvanted oradjuvanted influenza vaccines induced little or no cross-reactiveantibody response to 2009 H1N1 in any age group. Persons underthe age of 30 years had little evidence of cross-reactive antibodiesto the pandemic virus. However, a proportion of older adultshad 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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