Seasonal vaccine may not induce cross-reactive antibody response to influenza A (H1N1)

Cross-reactive antibody response by a factor of four may have occurred in fewer than 20% of adults who were vaccinated with recent seasonal influenza vaccines, according to results of a recent study.

The results indicated that individuals aged younger than 30 years demonstrated little evidence of cross-reactive antibodies to the current strain, but that a proportion of older adults may have pre-existing cross-reactive antibodies.

The researchers used a microneutralization assay to measure cross-reactive antibodies to influenza A (H1N1) in stored serum samples. The samples had been taken from individuals who had donated blood or were vaccinated with the recent seasonal vaccine or the 1976 swine influenza vaccine.

Among 107 individuals born after 1980, 4% had pre-existing cross-reactive antibody titers of 40 or more against the current pandemic strain. Among 115 individuals born before 1950, 34% had titers of 80 or more.

Among individuals vaccinated with seasonal trivalent inactivated influenza vaccines, an increase in the level of cross-reactive antibody to influenza A (H1N1) by a factor of four or more was observed in 0% of 55 children aged six months to nine years old, 12% to 22% of 231 of adults aged 18 to 64 years and in 5% or less of 113 adults older than 60.

There was no association between vaccines that were formulated with adjuvant and enhanced cross-reactive antibody response.

Substantially boosted cross-reactive antibodies to influenza A (H1N1) were observed in adults who had received the A/New Jersey/1976 swine influenza vaccine.

Hancock K et al. N Engl J Med. 2009;361:1945-1952.