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. IB was defined by month and county as the number of RSV detections per inhabitant. IB for 5 periods ante- and postnatally was calculated for each child. Outcome variables were maternally reported hospitalization for LRTI before age 6 months and LRTI before age 12 months. Logistic regression was used to estimate associations between IB and LRTI.
Results: The odds of hospitalization for LRTI rise with increased IB the first 3 months after birth. Low IB: odds ratio (OR) 1.17 (95% confidence interval [CI]: 0.98–1.39); medium IB: OR 1.42 (95% CI: 1.21–1.68); high IB: OR 2.51 (95% CI: 2.15–2.94). High IB 3–0 months before birth confers a lower odds of hospitalization for LRTI the first 6 months of life, OR 0.51 (95% CI: 0.43–0.61). Similar results were seen for maternally reported LRTI the first 12 months of life.
Conclusions: We find an association between real-time RSV infectious burden and LRTI in infancy: high burden before birth is protective and high burden after birth increases the risk.