A population-based study from British Columbia suggests that a substantial share of people already carry antibodies that may help protect against avian influenza A(H5N1), though levels vary by age and birth cohort, reflecting past exposure to different flu viruses.
In the study, led by researchers from the British Columbia Centre for Disease Control and published in The Journal of Infectious Diseases, researchers tested blood samples collected in August 2024 from 575 people aged 1 to over 80 years for cross-reactive neuraminidase antibody levels against A(H5N1).
Overall, 70% of participants had detectable antibodies against H5N1, with 45% meeting a moderate threshold, 32% meeting a slightly higher threshold, and 17% showing a high concentration of antibodies in the blood.
The highest antibody levels were seen among adults born from 1997 to 2003, who were school-aged children during the 2009 H1N1 flu pandemic. Similarly high levels were seen among adults born before 1947, who were likely exposed to early H1N1 influenza viruses.
Past pandemics may influence antibody levels
The researchers hypothesize that past flu pandemics exposed people in these age cohorts to the virus, which bolstered their antibody levels. “We interpret this variation within a unifying hypothesis incorporating both age and imprinting effects, emphasizing the role of historic influenza pandemics in expanding and refining the immune repertoire,” they write.
In contrast, antibody levels were lowest among young children born from 2015 to 2023, whom the authors called “the youngest and least influenza-experienced pediatric cohorts” and middle-aged adults born from 1957 to 1967, when H2N2 strains predominated.
The researchers caution that immunity can’t be inferred from antibody levels, but “patterns indicate higher likelihood of N1-based protection among those who experienced the highest attack rates during the 2009 pandemic… and among those born during the pre-1957 H1N1 era.”
The findings may help explain why recent H5N1 infections in US animal workers have generally been mild and why severe cases remain uncommon among older adults, though more investigation is warranted, the authors say.
