Seasonal flu shots are recommended for everyone ages 6 months and older, according to the Centers for Disease Control and Prevention.
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Women have a stronger immune response than men when given the flu vaccine, new research shows.
This may mean that vaccinated women are better protected against catching the flu than vaccinated men, although the new study did not look at this directly, the researchers said.
In the study, researchers examined the inflammatory responses of 53 women and 34 men following vaccination with a flu shot. Scientists found that men had a weaker response, or less inflammation in their bodies, than women after receiving the vaccine, and the response was weakest among some of the men who had the highest testosterone levels.
The finding "reinforces the message that there are major differences between men and women in terms of their immune systems," said study researcher Mark Davis, a professor of microbiology and immunology at the Stanford School of Medicine. "Like many other areas, it hasn't been that well explored."
Davis said the study does not make immediately clear whether men and women have different levels of flu protection after vaccination, but other research suggests they do. [6 Flu Vaccine Myths]
"There is a literature to say women have better responses in general to infectious diseases, including flu," Davis told LiveScience.
The reasons for the difference are not entirely known, but the researchers noted that testosterone suppresses inflammation.
It’s also possible there is a genetic component to people's flu responses, the researchers said. While the mechanism is not entirely clear, the researchers found the vaccine activated certain genes, and this activation predicted who would have the weakest flu shot response.
"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.