  Staphylococcus aureus nasal carriage increases risk of infection and has been
associated with lifestyle behavior and biological host characteristics. We used
social network analysis to evaluate whether contacts have the same S. aureus
genotype, or whether contagiousness is an indirect effect of contacts sharing
the same lifestyle or characteristics.
  The Fit Futures 1 study collected data on social contact among 1038 first
level students in the same high school district in Norway. S. aureus persistent
carriage was determined from two nasal swab cultures and genotype from
spa-typing of a positive throat swab culture. Bootstrap, t-tests, logistic
regression, and autocorrelation were used to evaluate social network influence
on host risk factors and S. aureus carriage.
  Both persistent carriage and spa-type were transmitted in the social network
(p<0.001). The probability of carriage increased by 3.7% and 5.0% for each
additional S. aureus positive friend, in univariable regression and
multivariable autocorrelation analysis respectively. Male sex was associated
with a 15% lower risk of transmission compared to women, although the
prevalence of carriage was higher for men (36% versus 24%). Medium physical
activity, medium and high alcohol-use, and normal-weight students had higher
number of contacts, and increased risk of transmission (p<0.002).
  We demonstrate direct social transmission of S. aureus in a general youth
population. Lifestyle factors are associated with risk of transmission
suggesting indirect social group effects from having more similar environmental
exposures. The predominance in carriage is determined by sex-specific
predisposing host characteristics as social transmission is less frequent than
in females. Better understanding of how social interactions influence S. aureus
carriage dynamics in the population is important for developing new preventive
measures.

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