Why do flu shots fail? Human immune system may be to blame

When patients catch the flu even after getting a flu shot, many physicians identify strain differences as the culprit. However, flu science is increasingly identifying another cause for failed flu protection: patients' own immune systems, according to STAT.

The publication identified a "growing body of evidence" that suggests sometimes immune systems do not respond appropriately to flu shots and fail to make antibodies against designated strains. The reason for this is "imprinting."

Imprinting, or "original antigenic sin," is the phenomenon that human immune systems will always be best equipped to defend against the first flu virus they were exposed to. For example, someone whose first influenza A infection was caused by an H3N2 virus will demonstrate a better immune response to this component of a flu vaccine than an H1N1 component, according to STAT.

"We've all been trained on different influenza viruses," Scott Hensley, PhD, an associate professor of microbiology and vaccine researcher at Philadelphia-based University of Pennsylvania, told STAT. "If you vaccinate 100 people, guess what? They're all going to respond differently. We think a large part of that is that we all have a different immunological imprint."

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