FIG 2.

Vaccine-induced protection against HRSV challenge in cotton rats at different ages. At the age of 2 or 9 months, cotton rats (n = 5 or 6 per group) were immunized with attenuated rHRSV (with doses indicated as TCID₅₀) or the mock control. One group of cotton rats at 9 months of age was immunized with wild-type HRSV (HRSV at 10⁴ TCID₅₀). Subsequently, all animals were challenged intranasally with 3 × 10⁵ TCID₅₀ of wild-type HRSV at 28 days postimmunization. Virus titers were analyzed in lungs at 5 days postchallenge. Less protection induced by rHRSV in old cotton rats was found in two independent experiments; data from one of these experiments are shown. * indicates that none of the challenged animals were free of HRSV at 5 days postchallenge.