Lung histology after SARS-CoV-2 inoculation of vaccinated aged rhesus macaques. Seven individual lung lobes were evaluated for each animal in each treatment group (19 NHPs total). (A–C) 20× magnification overview image per treatment group. Scale bar is 1 mm. (D–F) H&E staining of rectangular areas as indicated in A–C. (D) In the one-dose 10¹¹-vp Ad26.COV2.S group, minimal mononuclear cell interstitial infiltrate and minimal perivascular cuffing was observed. (E) In the two-dose 5 × 10¹⁰–vp Ad26.COV2.S group, multifocal small areas with minimal mixed-cell interstitial infiltrates (macrophages, scattered neutrophils, and lymphocytes) and minimal perivascular cuffing were observed. Alveolar lumina contained minimal macrophages, lymphocytes, and scattered neutrophils. (F) In the pooled control group animals, here represented by an animal of the Ad26.RSV.gLuc sham control group, focally extensive to diffuse lesions were observed with moderate mononuclear cell infiltrate in interstitium (macrophages, lymphocytes) and mild type II pneumocyte hyperplasia. Alveolar lumina contained edema, macrophages, lymphocytes, and scattered neutrophils. (G–I) SARS-CoV-2 −N immunohistochemistry (brown staining) of rectangular areas as indicated in A–C. Antigen-positive pneumocytes are marked with an asterisk (*). In the one-dose 10¹¹–vp Ad26.COV2.S group, only one out of six animals had a single lung lobe in which antigen-positive pneumocyte staining was observed. (G) Focal area with individual SARS-CoV-2 N-positive pneumocytes (*) in the left caudal lung lobe. (H) Absence of SARS-CoV-2 N-positive pneumocytes in all lung lobes from all animals in the two-dose 5 × 10¹⁰–vp Ad26.COV2.S group. (I) Multifocal minimal to moderate numbers of SARS-CoV-2 N-positive pneumocytes (*) in seven out of eight pooled control animals. D–I are 200× magnification; scale bar is 100 µm.