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Preexisting Immunity to the 2009 H1N1 Virus

Very few individuals younger than 30 have protective levels of cross-reactive neutralizing antibodies against this virus.

The 2009 pandemic influenza A (H1N1) virus has several genetically unique features, and public health experts are concerned that this virus could cause millions of human infections during the upcoming "flu season." To predict disease activity related to the 2009 H1N1 virus, authorities need information about preexisting immunity to this pathogen in the population.

Toward this end, CDC investigators tested stored sera from individuals of various ages who lived in the U.S. or Europe and who participated in vaccine trials or had serum samples collected anonymously for blood donation or other purposes. They used a microneutralization assay, a hemagglutination-inhibition assay, or both, to measure levels of cross-reactive neutralizing antibodies against the 2009 H1N1 virus. A serum hemagglutination-inhibition antibody titer of 40, which has been associated with a 50% reduction in risk for infection or disease from influenza viruses in human populations, or the corresponding microneutralization titer (40 for children; 80 or 160 for adults, depending on age), was defined as protective.

Among 100 children aged 6 months to 9 years who had participated in earlier seasonal influenza vaccine trials, only 1 had protective antibody titers against the 2009 H1N1 virus before vaccination, and only 4 had them afterward. The results for the 393 adults who had participated in seasonal vaccine trials varied by age. Among those aged 18 to 64, levels of protection against the 2009 H1N1 virus were 6% to 7% prevaccination and 7% to 25% postvaccination. For those aged 60, these levels were 8% to 33% before vaccination and 8% to 43% afterward.

For the individuals whose serum samples were collected anonymously, 39 of 115 (34%) who were born before 1950 had titers 80 for antibodies against the 2009 H1N1 virus, whereas only 4 of 107 (4%) of those born after 1980 had titers 40. Among the 83 adults who had received the swine flu (A/NJ/76) vaccine in 1976, 63% had protective levels of antibodies against the 2009 H1N1 virus.

Comment: The results of this serologic survey are disturbing. The degree of preexisting immunity across all age groups is fair, at best, substantiating the concern that a very active "flu season" is likely during the coming months, particularly among individuals aged <30.

— Larry M. Baddour, MD

Published in Journal Watch Infectious Diseases September 16, 2009

Citation(s):

Hancock K et al. Cross-reactive antibody responses to the 2009 pandemic H1N1 influenza virus. N Engl J Med 2009 Sep 10; [e-pub ahead of print]. (http://dx.doi.org/10.1056/NEJMoa0906453)

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• Protective antibody titre by subclinical infections H1N1
  Dr.Harshad K.Takvani Takvani, Takvani Children Hospital, 4 Jan 2010 1:53 PM EST
  Dear sir/Madam, I am seeing many patients with viral respiratory infection in this season. One of my patient was proved... [more]