Nineteen percent of volunteers also produced antibodies that neutralized the 2009 pandemic strain and blocked it from infecting cells. In comparison, more than 67 percent of volunteers had antibodies that neutralized the 2008-09 seasonal H1N1 strain.

Those vaccinated in 1976 were more likely to make neutralizing antibodies against the new pandemic strain. More than 17 percent of the 1976-vaccine group made such antibodies in large quantities. Only about 4 percent of those who had not received the 1976 shot had comparable levels of antibody production. The difference between the two groups was statistically significant, meaning it was unlikely chance alone explained the result.

The work reflects ongoing efforts to understand why the current pandemic flu has taken a greater toll on children and young adults than on those ages 65 and older. In this study, researchers focused on older individuals to better gauge the impact of the 1976 "swine flu" shot or possible childhood exposure to flu viruses similar to the current pandemic strain. McCullers said those viruses last circulated in the 1930s and 1940s.

The unexpectedly robust immune response mounted by all the volunteers suggests that routine vaccination against seasonal flu might confer a broader-than- realized protection, McCullers said. The St. Jude volunteers included many health care workers who are vaccinated annually against flu.

Source: St. Jude Children's Research Hospital