Infectious burden of respiratory syncytial virus in relation to time of birth modifies the risk of lower respiratory tract infection in infancy: the Norwegian Mother and Child Cohort.

Objective: The risk of respiratory syncytial virus (RSV) infection according to calendar month of birth has previously been demonstrated. We hypothesize that the real-time infectious burden (IB) of RSV, in relation to time and county of birth, modifies the risk of lower respiratory tract infection (LRTI) in infants, an association that has not been investigated previously, and may be used as an alternative measure of RSV risk when drafting RSV immunoprophylaxis guidelines.

Methods: Thirty thousand eighty-seven pregnancies were included from July 1, 2003, to July 31, 2006.