Exacerbation of pulmonary pathology in hMPV challenged cotton rats vaccinated with FI-hMPV. Medium power photomicrographs of cotton rat lungs at various days after challenge with human metapneumovirus. Each frame is centered on a bronchiole that is accompanied by a blood vessel. Pulmonary histopathology by days post-challenge (4, 7, and 10) with human metapneumovirus in cotton rats with a primary infection, reinfection, or that received mock or formalin-inactivated metapneumovirus vaccine before challenge. Pathology score from 0 to 4 scale in severity of infiltration of inflammatory cells using four parameters (Peribronchiolitis, Perivasculitis, Interstitial Pneumonitis, and Alveolitis).