Nonadjuvanted Influenza Vaccines May Not Protect Against 2009 H1N1

From Medscape Medical News
Laurie Barclay, MD

September 11, 2009 — Recent seasonal nonadjuvanted influenza vaccines may not protect against pandemic H1N1 virus (2009 H1N1), according to the results of a study reported online September 10 issue in the New England Journal of Medicine.

"A new pandemic influenza A (H1N1) virus has emerged, causing illness globally, primarily in younger age groups," write Kathy Hancock, PhD, from the Influenza Division, National Center for Immunization and Respiratory Diseases, the Centers for Disease Control and Prevention, Atlanta, Georgia, and colleagues. "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."

The investigators used a microneutralization assay to quantify titers of antibodies cross-reactive to 2009 H1N1 in stored serum samples from individuals who either donated blood or who received recent seasonal or 1976 swine influenza vaccines.

Preexisting cross-reactive antibody titers of 40 or more against 2009 H1N1 were present in 4 (4%) of 107 persons born after 1980. In contrast, 39 (34%) of 115 persons born before 1950 had titers of 80 or more.

The level of cross-reactive antibody to 2009 H1N1 increased by 4-fold or greater after receipt of seasonal trivalent inactivated influenza vaccines in none of 55 children aged 6 months to 9 years, in 12% to 22% of 231 adults aged 18 to 64 years, and in 5% or less of 113 adults aged 60 years or older. Cross-reactive antibody responses were not improved by seasonal vaccines formulated with adjuvant. In adults, administration of the A/New Jersey/1976 swine influenza vaccine substantially increased cross-reactive antibodies to 2009 H1N1.

"Vaccination with recent seasonal nonadjuvanted influenza vaccines induced little or no cross-reactive antibody response to 2009 H1N1 in any age group," the study authors write. "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."

Limitations of this study include the relatively small number of samples from pediatric trials.

"It remains clear that optimal protection against 2009 H1N1 in persons of all ages will be achieved with the development of a strain-specific pandemic vaccine," the study authors write. "Whether a one-dose or a two-dose vaccine regimen is needed to adequately immunize various age groups and whether the use of adjuvants will broaden the immune response against 2009 H1N1 if drifted strains emerge or provide dose-sparing benefits will ultimately be determined by the results of clinical studies that are now underway."

N Engl J Med. Published online September 10, 2009.