Inflammatory reactivity to the influenza vaccine is associated with changes in automatic social behavior.

Recent evidence suggests differential patterns of social behavior following an inflammatory challenge, such that increases in inflammation may not uniformly lead to social withdrawal. Indeed, increases in inflammation have been associated with enhanced self-reported motivation to approach a specific close other, and greater neural sensitivity to positive social cues. However, no known studies have examined the association between inflammation in response to an inflammatory challenge and social behavior in humans, nor has past research examined specifically how approach and withdrawal behavior may differ based on whether the target is a close other or stranger. To address this, 31 participants (ages 18-24) received the influenza vaccine to elicit a low-grade inflammatory response. The morning before and approximately 24 h after the vaccine, participants provided a blood sample and completed a computer task assessing automatic (implicit) approach and withdrawal behavior toward a social support figure and strangers. Greater increases in the inflammatory cytokine interleukin-6 (IL-6) in response to the vaccine were associated with an increase in accuracy in avoiding strangers and a decrease in accuracy in approaching them. Increases in IL-6 were also associated with a decrease in reaction time to approach a support figure, but only when controlling for baseline IL-6 levels. There were no associations between change in IL-6 and changes in self-reported motivation to engage in social behavior with either close others, or strangers. Together, these findings reveal that increases in inflammation following the influenza vaccine are associated with automatic social behavior, especially behavior suggesting avoidance of unfamiliar social targets and ease in approaching a support figure. These data add to the growing literature suggesting that the association between inflammation and social behavior includes both social withdrawal and social approach, depending on the specific target.