OBJECTIVE. We estimated differences in the severity of respiratory syncytial virus infection attributable to genetic and environmental factors.

METHODS. Record linkage data on hospitalizations attributable to respiratory syncytial virus infection were gathered on all twins (12346 pairs) born in Denmark between 1994 and 2003. Latent-factor models of genetic and environmental effects were fitted to the observed data by using maximal likelihood methods.

RESULTS. Identical twins resembled each other significantly more than did fraternal twins for respiratory syncytial virus hospitalization (concordance rate: 0.66 vs 0.53), which suggests genetic influences on disease severity. Genetic factors accounted for 16%, family environment for 73%, and nonshared environment for 11% of the individual susceptibility to develop severe respiratory syncytial virus infection.

CONCLUSIONS. The severity of respiratory syncytial virus infection is determined partly by genetic factors. This result should stimulate the search for genetic markers of disease severity.

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