Older adults exposed to seasonal flu strains that circulated before 1968 may be less vulnerable to bird flu as they are likely to have antibodies that recognize and attack the H5N1 avian flu virus, researchers reported in Nature Medicine.

Younger adults and children do not have such antibodies and might benefit more from bird flu vaccines, even if the vaccines are not specifically tailored to the current strain circulating in birds and cattle, the researchers said.

Testing blood samples from 157 people born between 1927 and 2016, they found that an individual's birth year was closely linked to the amount of H5N1-fighting antibodies in their blood.

Early childhood influenza exposures can elicit immune responses that last a lifetime, study leader Scott Hensley of the University of Pennsylvania said in a statement.

Antibody responses that were primed by the seasonal flu viruses H1N1 and H3N2 "decades ago can cross-react to H5N1 avian viruses circulating today,” Hensley said of their findings.

Most of these antibodies cannot prevent infections, but they will likely limit severity of disease if there is a bird flu pandemic, he added.

When a separate group of 100 volunteers received a vaccine designed to target an older bird flu strain that circulated in 2004, antibodies that could recognize modern bird flu strains increased slightly in older adults but “substantially” in children, the researchers reported.

In the event of a pandemic, younger individuals are therefore more likely to benefit from bird flu vaccines, even those not tailored to a current strain, they said.

The U.S. Centers for Disease Control and Prevention currently considers the risk from bird flu to the general public to be low. But scientists worry the virus could mutate in ways that will allow it to spread more easily among people.