Scientists say some flu-shot formulations activate our dendritic cells, encouraging our immune systems to fight harder against the virus. The researchers published their findings in the journal Science Translational Medicine.

Guarding against the flu is a tricky thing. Every flu season, public health officials have to predict which strains will predominate. But sometimes, even when they’ve got that right, the vaccine just doesn’t work as well as it should. During 2009, for example, one of the available vaccines (MIV-09) was 35 percent less effective than another (TIV-09)—despite the fact that both were made by the same company. This wasn’t an issue of quality control. Something else was going on.

One group of immunologists had a theory.

Dendritic cell. Image Credit: Sriram Subramaniam, National Cancer Institute (NCI) and Donny Bliss, National Library of Medicine (NLM) via Wikimedia Commons // Public Domain

Dendritic cells (so called for their branching protrusions, or dendrites) are the messengers of the immune system. They’re found on your skin and in your stomach, lungs, intestines, and nose, patrolling the borders between the inside and outside of your body. As security guards, they’re pretty sharp, responding differently to different types of intruders.

Could it be that dendritic cells were helping one vaccine work or hindering another?

To find out, the researchers cultured human cells in the lab, then exposed them to the two vaccines while watching to see how the dendritic cells would respond. The trivalent vaccine aims to protect against three types of influenza strains; the monovalent vaccine, one.

Carla Schaffer / S. Athale / Science Translational Medicine (2017).

Sure enough, the TIV-09 that had fared so much better in 2009 inspired more cooperation from dendritic cells. Dendritic cells treated with MIV-09 were less likely to respond, even when they’d already been activated.

Further studies will be needed to confirm these tests in actual human bodies, but these findings are a good start.