How Your Birth Year Could Affect Your Ability to Survive a Flu Epidemic

Updated 12/5/2016

Your birth year may predict your chances of survival in an influenza A virus (IAV) pandemic, suggest the results of a recent study published in the journal Science.

“Our findings show that major patterns in zoonotic IAV epidemiology, previously attributed to patient age, are, in fact, driven by birth year,” Katelyn M. Gostic, the University of California, Los Angeles, and colleagues, wrote. “IAV strains circulating during an individual’s childhood confer long-term protection against novel hemagglutinin [HA] subtypes from the same phylogenetic group.”

The main antigenic determinants for IAV susceptibility are the virus’s two surface glycoproteins, HA and neuraminidase. IAVs are classified into two major groups, based on their surface HA glycoproteins: HA group 1 includes H1, H2, and avian H5 subtypes, and HA group 2 includes H3 and avian H7 subtypes. H5N1 and H7N9 are two subtypes of avian-origin IAV that have caused serious infections and deaths in people. However, the differences in age distribution associated with outbreaks of these subtypes have perplexed scientists: although H5N1 has predominantly affected children, H7N9 has mainly affected older adults.

In order to explain these differences, the researchers analyzed data on all human H5N1 and H7N9 infections and found a link between birth year and the level of immune protection a person had against different influenza subtypes. “Our analysis of human cases of H5N1 and H7N9 revealed strong evidence that childhood HA imprinting indeed provides profound, lifelong protection against severe infection and death from these viruses,” the authors wrote.

According to the authors, the subtype of IAV that a person first encounters as a child leaves a permanent “immunological imprint” on the immune system, providing strong protection against similar strains and weaker protection against less closely related subtypes—a concept known as antigenic seniority. They showed that individuals who experienced first IAV infections with the seasonal H3 subtype were less susceptible to the potentially fatal avian influenza H7N9 virus. In addition, they found that older individuals who were exposed to H1 or H2 subtype viruses as children were less susceptible to avian H5N1-bearing viruses. People born before 1968 were 75% less likely to suffer a severe case of H5N1 or H7N9 influenza after being infected with that virus, and were 80% less likely to die, than those born after 1968 were. In contrast, those born after 1968 had higher levels of protection against H7N9 than those born before 1968.

These findings show that antigenic seniority extends across IAV subtypes, introducing a previously unrecognized generational structure to influenza epidemiology. Therefore, immune imprinting has implications for public health and highlights that influenza virulence is influenced by both virus and host.

The results of this study allow scientists to predict age distributions of severe disease for future pandemics and show that the pandemic potential of a new strain increases yearly when a group-mismatched HA subtype dominates seasonal influenza circulation, Gostic and colleagues concluded. “These findings open new frontiers for rational pandemic risk assessment.”

Dr. Parry graduated from the University of Liverpool, England, in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC, where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals, and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.