"There were a set of genes that were activated, or up-regulated, in men, and that showed the difference," said David Furman, a postdoctoral researcher in Davis's lab and first author on the study. "It turns out that those having the highest testosterone levels and the expression of these gene signatures, they do very, very bad" with their immune response to the vaccine.
In their paper, the authors speculated that an evolutionary reason may explain why men would benefit from suppressed immune systems. In evolutionary terms, men may have experienced more trauma than women, so they may have benefitted from a less-active immune system.
"It also turns out that testosterone suppresses inflammation and that inflammation can be a problem in lots of circumstances. It's a necessary part of immunity … but if it gets out of hand, it can kill you," Davis said.
Gender differences in immunity make for an area ripe for study, he said. Health researchers want to know why women have much higher rates of certain immune diseases, and why during pregnancy those diseases may go into remission. Davis said he hopes his study provides some fodder for subsequent research in the area.
In the future, the new study might suggest some means of improving flu shots, perhaps by adding an ingredient to shots given to men, he said.
"There might be a way to disconnect the testosterone suppression to, say, getting better immunity," Davis said.
The study is being published today (Dec. 23) in the journal Proceedings of the National Academy of Sciences.
More From LiveScience:
Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
This story originally appeared on LiveScience.com.