Low vaccine effectiveness is a concern every year, especially this time of year when flu activity is typically at its highest levels. The problem is often blamed on the flu strains chosen for the vaccine and/or vaccine production issues related to the use of eggs. Now researchers from the University of Chicago Medical Center say that immune history with the flu could also influence a person’s response to the vaccine.
In 2012-13, the H3N2 component of the flu vaccine was effective in just 39% of people. That season, public health officials believed that adaptations in egg-grown vaccines were the problem. But in a new study published today in Clinical Infectious Diseases, researchers show that poor immune responses, not egg adaptions, may explain the low effectiveness of the vaccine that year.
"Egg adaptations have variable effects," said Sarah Cobey, assistant professor of ecology and evolution at UChicago and lead author of the study. "Sometimes they matter and sometimes they don't, but what seems to make the most difference is immune history."
What's at play seems to be a phenomenon known as "original antigenic sin." Flu vaccines are designed to get the immune system to produce antibodies that recognize the specific strains of the virus someone may encounter in a given year. These antibodies target unique sites on the virus, and latch onto them to disable it. Once the immune system already has antibodies to target a given site on the virus, it preferentially reactivates the same immune cells the next time it encounters the virus.
This is efficient for the immune system, but the problem is that the virus changes ever so slightly from year to year. The site the antibodies recognize could still be there, but it may no longer be the crucial one to neutralize the virus. Antibodies produced from our first encounters with the flu, either from vaccines or infection, tend to take precedence over ones generated by later inoculations. So even when the vaccine is a good match for a given year, if someone has a history with the flu, the immune response to a new vaccine could be less protective.