population based study British Columbia researchers suggest that a significant proportion of people already have antibodies that may help protect against avian influenza A (H5N1), but levels vary by age and birth cohort and reflect past exposure to different influenza viruses.

The study was led by researchers at the British Columbia Center for Disease Control. infectious disease journalResearchers tested blood samples collected in August 2024 from 575 people aged 1 to over 80 years for levels of cross-reactive neuraminidase antibodies against A(H5N1).

Overall, 70% of participants had detectable antibodies to H5N1, 45% met a moderate threshold, 32% met a slightly high threshold, and 17% had high levels of antibodies in their blood.

Antibody levels were highest in adults born between 1997 and 2003, who were school-age children during the 2009 H1N1 influenza pandemic. Similarly, high levels were found in adults born before 1947, who were likely exposed to early H1N1 influenza viruses.

Past pandemics may affect antibody levels

Researchers hypothesize that past influenza pandemics may have exposed people in these age groups to the virus, leading to increased antibody levels. “We interpret this variation within a unifying hypothesis that incorporates both age and imprinting effects, highlighting the role of historical influenza pandemics in expanding and refining the immune repertoire,” the researchers wrote.

In contrast, antibody levels were lowest in infants born between 2015 and 2023, what the authors called “the youngest and least influenza-experienced pediatric cohort,” and middle-aged adults born between 1957 and 1967, when the H2N2 strain was predominant.

The researchers cautioned that immunity cannot be inferred from antibody levels, but “patterns suggest a higher likelihood of N1-based protection among those who experienced the highest attack rates during the 2009 pandemic and among those born in the pre-1957 H1N1 era.”

The findings may help explain why recent H5N1 infections among U.S. animal workers have generally been mild, and why severe cases remain rare among older adults, but further investigation is needed, the authors say.