Protective Immunity of the Primary SARS-CoV-2 Infection Reduces Disease Severity Post Re-Infection with Delta Variants in Syrian Hamsters

Abstract

The Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) Delta variant has evolved to become the dominant SARS-CoV-2 lineage with multiple sub-lineages and there are also reports of re-infections caused by this variant. We studied the disease characteristics induced by the Delta AY.1 variant and compared it with the Delta and B.1 variants in Syrian hamsters. We also assessed the potential of re-infection by these variants in Coronavirus disease 2019 recovered hamsters 3 months after initial infection. The variants produced disease characterized by high viral load in the respiratory tract and interstitial pneumonia. The Delta AY.1 variant produced mild disease in the hamster model and did not show any evidence of neutralization resistance due to the presence of the K417N mutation, as speculated. Re-infection with a high virus dose of the Delta and B.1 variants 3 months after B.1 variant infection resulted in reduced virus shedding, disease severity and increased neutralizing antibody levels in the re-infected hamsters. The reduction in viral load and lung disease after re-infection with the Delta AY.1 variant was not marked. Upper respiratory tract viral RNA loads remained similar after re-infection in all the groups. The present findings show that prior infection could not produce sterilizing immunity but that it can broaden the neutralizing response and reduce disease severity in case of reinfection.

Keywords: Delta AY.1; SARS-CoV-2; Syrian hamsters; pathogenicity; re-infection.

Figure 1

Study design. (a) Summary of the Delta AY.1 vs. Delta and B.1 pathogenicity study. (b) Summary of the reinfection process.

Figure 2

Body weight loss and immune response in hamsters post infection. (a) Line graph depicting body weight loss in hamsters post first infection. (Delta vs. Delta AY.1 on 6 DPI, p = 0.0248 and Delta vs. B.1 on 6 DPI, p = 0.0082). The error bars depict the standard deviation (SD). (b) Scatter plot depicting IgG ELISA titre in hamsters post first infection (n = 5) on days 7 and 14. Scatter plot depicting (c) PRNT50 titres against variants in Delta AY.1-infected hamsters (n = 5), Delta-infected hamsters, (p = 0.0066 (Delta vs. Beta), Mann–Whitney test, n = 5) and B.1-infected hamsters (p = 0.0080 B.1 vs. Beta, Mann–Whitney test, n = 5). The bars represent the mean and the error bars depict the SD. Limit of detection of assay is depicted as the dotted line and ** represents p value < 0.001.

Figure 3

SARS-CoV-2 viral RNA shedding in hamsters after infection. Scatter plot depicting viral gRNA load in the (a) throat swabs (c) nasal washes and (e) faeces of hamsters post infection, (Mann–Whitney test, n = 7). Scatter plot depicting viral sgRNA load in (b) throat swabs, (d) nasal washes and (f) faeces of hamsters post infection, (Mann–Whitney test, n = 7). The bars represent the mean and the error bars depict the standard deviation. The p values < 0.05, <0.001 and <0.0001 are represented as *, ** and *** respectively.

Figure 4

Body weight loss and immune response in hamsters post re-infection. (a) Line graph representing percent body weight change in hamsters on 2, 4 and 6 DPI. The error bars depict the standard deviation (SD). (b) Scatter plot depicting IgG ELISA titre in hamsters before and after re-infection on days 0 and 7. PRNT50 titres in (c) Delta-, (d) Delta AY.1- and (e) B.1-infected hamsters before and after re-infection on 7 DPI. The bars represent the means and the error bars depict the SDs. Limit of detection of the assay is shown by the dotted line.

Figure 5

Histopathological changes observed in lungs after primary infection and re-infection. Lung sections from hamsters infected with the B.1 variant showing (a,b) Diffuse interstitial pneumonia and (c) large foci of alveolar interstitial thickening, inflammatory cell infiltration, loss of bronchial epithelium and congestion in the parenchyma on 7 DPI after primary infection, as revealed by H&E staining, scale bar = 100 µm. Lung sections from hamsters infected with the Delta AY.1 variant showing (d) diffuse alveolar capillary engorgement, (e) peribronchial inflammatory cell infiltration and (f) diffuse alveolar capillary engorgement and hemorrhages on 7 DPI after primary infection, H&E, scale bar = 100 µm. Lung sections from hamsters infected with the Delta variant showing (g) diffuse alveolar damage with hemorrhage in the parenchyma, (h) diffuse alveolar septal thickening and exudation and (i) alveolar interstitial thickening, congestion and peribronchial inflammatory cell infiltration on 7 DPI after primary infection, H&E, scale bar = 100 µm. Lung sections from hamsters re-infected with the B.1 variant showing (j,k) a small amount of focal alveolar interstitial thickening and (l) alveolar interstitial thickening and congestion on 7 DPI after re-infection, H&E, scale bar = 100 µm. Lung sections from hamsters re-infected with the Delta AY.1 variant showing (m–o) diffuse alveolar interstitial thickening, congestion and inflammatory cell infiltration on 7 DPI after re-infection, H&E, scale bar = 100 µm. Lung sections from hamsters infected with the Delta variant showing (p) diffuse congestive changes, (q) alveolar septal thickening, peribronchial inflammatory cell infiltration and loss of bronchial epithelium and (r) diffuse engorgement, hemorrhages and peribronchial inflammatory cell filtration on 7 DPI post re-infection, H& E, scale bar = 100 µm. (s) Scatter plot depicting cumulative lung histopathology scores in hamsters post primary infection and re-infection with Delta, Delta AY.1 and B.1 on 7 DPI. The samples were scored on a scale of 0 to +4 for vascular changes, bronchial changes, alveolar parenchymal changes and inflammatory cellular infiltration. The bars represent the means and the error bars the standard deviations.

Figure 6

SARS-CoV-2 viral RNA shedding in hamsters after re-infection. Superimposed scatter plot depicting viral gRNA and sgRNA load in (a) throat swabs (p = 0.0286, Mann–Whitney test, n = 4) and (b) nasal washes (p = 0.0286, Mann–Whitney test, n = 4) and (c) faeces of the Delta variant group; the (d) throat swabs, (e) nasal washes and (f) faeces of the Delta AY.1 group; and (g) the throat swabs (p = 0.0286, Mann–Whitney test, n=4), (h) nasal washes (p = 0.0286, Mann–Whitney test, n = 4) and (i) faeces (p = 0.0286, Mann–Whitney test, n = 4) of the B.1-infected hamsters during primary infection and re-infection on 7 DPI. The bars represent the means and the error bars the standard deviations. p value < 0.05 is represented as *.

Figure 7

SARS-CoV-2 viral RNA load in organs after re-infection. Superimposed scatter plot depicting viral gRNA and sgRNA loads in the nasal turbinates and lungs of (a) Delta-, (b) Delta AY.1- and (c) B.1- (p = 0.0286, Mann–Whitney test, n = 4) infected hamsters during primary infection and re-infection on 7 DPI. The bars represent the means and the error bars the standard deviations. p value < 0.05 is represented as *.